A walk in the PARC: developing and implementing 21st century chemical risk assessment in Europe

Current approaches for the assessment of environmental and human health risks due to exposure to chemical substances have served their purpose reasonably well. Nevertheless, the systems in place for different uses of chemicals are faced with various challenges, ranging from a growing number of chemicals to changes in the types of chemicals and materials produced. This has triggered global awareness of the need for a paradigm shift, which in turn has led to the publication of new concepts for chemical risk assessment and explorations of how to translate these concepts into pragmatic approaches. As a result, next-generation risk assessment (NGRA) is generally seen as the way forward. However, incorporating new scientific insights and innovative approaches into hazard and exposure assessments in such a way that regulatory needs are adequately met has appeared to be challenging. The European Partnership for the Assessment of Risks from Chemicals (PARC) has been designed to address various challenges associated with innovating chemical risk assessment. Its overall goal is to consolidate and strengthen the European research and innovation capacity for chemical risk assessment to protect human health and the environment. With around 200 participating organisations from all over Europe, including three European agencies, and a total budget of over 400 million euro, PARC is one of the largest projects of its kind. It has a duration of seven years and is coordinated by ANSES, the French Agency for Food, Environmental and Occupational Health & Safety.

Is Oral Bilingualism an Advantage for Word Learning in Children With Hearing Loss?

Purpose The purpose of this study was to examine whether oral bilingualism could be an advantage for children with hearing loss when learning new words. Method Twenty monolingual and 13 bilingual children with hearing loss were compared with each other and with 20 monolingual and 20 bilingual children with normal hearing on receptive vocabulary and on three word-learning tasks containing nonsense words in familiar (English and Spanish) and unfamiliar (Arabic) languages. We measured word learning on the day of the training and retention the next day using an auditory recognition task. Analyses of covariance were used to compare performance on the word learning tasks by language group (monolingual vs. bilingual) and hearing status (normal hearing vs. hearing loss), controlling for age and maternal education. Results No significant differences were observed between monolingual and bilingual children with and without hearing loss in any of the word-learning task. Children with hearing loss performed more poorly than their hearing peers in Spanish word retention and Arabic word learning and retention. Conclusions Children with hearing loss who grew up being exposed to Spanish did not show higher or lower word-learning abilities than monolingual children with hearing loss exposed to English only. Therefore, oral bilingualism was neither an advantage nor a disadvantage for word learning. Hearing loss negatively affected performance in monolingual and bilingual children when learning words in languages other than English (the dominant language). Monolingual and bilingual children with hearing loss are equally at risk for word-learning difficulties and vocabulary size matters for word learning.

Development of an Instrument to Evaluate Audiologic Counseling Skills

This study describes the development of an instrument designed to evaluate audiologic counseling skills. In simulated counseling sessions, a trained actor portrayed a parent, and ten graduate audiology students role-played counseling sessions as audiologists informing the “parent” that her infant has a hearing loss. The ten sessions were videotaped, and three raters viewed the taped sessions while evaluating counseling skills with a new evaluation tool, the Audiologic Counseling Evaluation (ACE). The ACE was found to have excellent internal reliability (α = .91) and moderate-to-good inter-rater reliability. Raters' subjective evaluations of the tool were generally positive, and students' evaluations of the simulated counseling experience were overwhelmingly so. This instrument can be used by audiology faculty and clinical instructors to help students improve their counseling skills before interacting with parents. It can also be used in clinical settings for professional development by way of self- and peer-evaluation. Este estudio describe el desarrollo de un instrumento designado para la evaluación las destrezas en la consejería audiológica. En sesiones simuladas de consejería, un actor entrenado actuó como un progenitor, y 10 estudiantes graduados de audiología actuaron en dichas sesiones como audiólogos que informaban al "padre" que su niño tenía una pérdida auditiva. Las 10 sesiones fueron filmadas en video, y tres observadores calificaron las sesiones grabadas en tanto que evaluaban las destrezas de consejería con una nueva herramienta de evaluación, la Evaluación de Consejería Audiológica (ACE). Se encontró que la ACE tenía una excelente confiabilidad interna (α = .91) y una confiabilidad moderada a buena entre los evaluadores. Las evaluaciones subjetivas de la herramienta por parte de los jueces fueron positivas, y también las evaluaciones de los estudiantes simulando la experiencia de consejería. Este instrumento puede ser utilizado por profesores de audiología y por instructores clínicos para ayudar a los estudiantes a mejorar sus destrezas de consejería antes de interactuar con los padres. Puede utilizarse en situaciones clínicas para desarrollo profesional por medio de auto-evaluaciones y evaluaciones entre estudiantes.

DNAscan: personal computer compatible NGS analysis, annotation and visualisation

BACKGROUND

Next Generation Sequencing (NGS) is a commonly used technology for studying the genetic basis of biological processes and it underpins the aspirations of precision medicine. However, there are significant challenges when dealing with NGS data. Firstly, a huge number of bioinformatics tools for a wide range of uses exist, therefore it is challenging to design an analysis pipeline. Secondly, NGS analysis is computationally intensive, requiring expensive infrastructure, and many medical and research centres do not have adequate high performance computing facilities and cloud computing is not always an option due to privacy and ownership issues. Finally, the interpretation of the results is not trivial and most available pipelines lack the utilities to favour this crucial step.

RESULTS

We have therefore developed a fast and efficient bioinformatics pipeline that allows for the analysis of DNA sequencing data, while requiring little computational effort and memory usage. DNAscan can analyse a whole exome sequencing sample in 1 h and a 40x whole genome sequencing sample in 13 h, on a midrange computer. The pipeline can look for single nucleotide variants, small indels, structural variants, repeat expansions and viral genetic material (or any other organism). Its results are annotated using a customisable variety of databases and are available for an on-the-fly visualisation with a local deployment of the gene.iobio platform. DNAscan is implemented in Python. Its code and documentation are available on GitHub: https://github.com/KHP-Informatics/DNAscan . Instructions for an easy and fast deployment with Docker and Singularity are also provided on GitHub.

CONCLUSIONS

DNAscan is an extremely fast and computationally efficient pipeline for analysis, visualization and interpretation of NGS data. It is designed to provide a powerful and easy-to-use tool for applications in biomedical research and diagnostic medicine, at minimal computational cost. Its comprehensive approach will maximise the potential audience of users, bringing such analyses within the reach of non-specialist laboratories, and those from centres with limited funding available.

Effect of phonotactic probability and neighborhood density on word-learning configuration by preschoolers with typical development and specific language impairment

PURPOSE

In this study, the authors assessed the effects of phonotactic probability and neighborhood density on word-learning configuration by preschoolers with specific language impairment (SLI) and typical language development (TD).

METHOD

One hundred thirty-one children participated: 48 with SLI, 44 with TD matched on age and gender, and 39 with TD matched on vocabulary and gender. Referent identification and naming were assessed in a computer-based learning context.

RESULTS

For referent identification, preschoolers with TD benefited from high phonotactic probability, and the younger group also benefited from low neighborhood density. In contrast, the SLI group benefited only from high neighborhood density. For naming, older preschoolers with TD benefited most from low-density words, younger preschoolers with TD benefited most from words with high phonotactic probability, and the SLI group showed no advantage.

CONCLUSION

Phonotactic probability and neighborhood density had different effects on each group that may be related to children's ability to store well-specified word forms and to the size of their extant lexicon. The authors argue that cross-study comparisons of word learning are needed; therefore, researchers should describe word, referent, and learner characteristics and the learning context and should situate their studies in a triggering → configuration + engagement model of word learning.

Cav1.2 and Cav1.3 L-type calcium channels regulate dopaminergic firing activity in the mouse ventral tegmental area

Dopaminergic projections from the ventral tegmental area (VTA) constitute the mesolimbocortical system that underlies addiction and psychosis primarily as a result of increased dopaminergic transmission. Dopamine release is spike dependent. L-type calcium channels (LTCCs) play an important role in regulating firing activities, but the contribution of specific subtypes remains unclear. This article describes different functions of Cav1.2 and Cav1.3 subtypes in regulating firing properties with two transgenic mouse strains. For basal firing, Cav1.3-deficient (Cav1.3(-/-)) mice had a lower basal firing frequency. The dihydropyridine (DHP) channel blocker nifedipine reduced single-spike firing in mice expressing DHP-insensitive Cav1.2 channels (Cav1.2DHP(-/-) mice), confirming the significant contribution from the Cav1.3 subtype in basal firing. Moreover, the DHP channel activator (S)-(-)-Bay K8644 and the non-DHP channel activator FPL 64176 converted firing patterns from single spiking to bursting in Cav1.2DHP(-/-) mice. Nifedipine inhibited burst firing induced by both activators, suggesting that Cav1.3 also serves an essential role in burst firing. However, FPL 64176 also induced bursting in Cav1.3(-/-) mice. These results indicate that the Cav1.3 subtype is crucial to regulation of basal single-spike firing, while activation of both Cav1.2 and Cav1.3 can support burst firing of VTA neurons.

Effects of tubing length and coupling method on hearing threshold and real-ear to coupler difference measures

PURPOSE

This tutorial demonstrates the effects of tubing length and coupling type (i.e., foam tip or personal earmold) on hearing threshold and real-ear-to-coupler difference (RECD) measures.

METHOD

Hearing thresholds from 0.25 kHz through 8 kHz are reported at various tubing lengths for 28 normal-hearing adults between the ages of 22 and 31 years. RECD values are reported for 14 of the adults. All measures were made with an insert earphone coupled to a standard foam tip and with an insert earphone coupled to each participant's personal earmold.

RESULTS

Threshold and RECD measures obtained with a personal earmold were significantly different from those obtained with a foam tip on repeated measures analyses of variance. One-sample t tests showed these differences to vary systematically with increasing tubing length, with the largest average differences (7-8 dB) occurring at 4 kHz.

CONCLUSIONS

This systematic examination demonstrates the equal and opposite effects of tubing length on threshold and acoustic measures. Specifically, as tubing length increased, sound pressure level in the ear canal decreased, affecting both hearing thresholds and the real-ear portion of the RECDs. This demonstration shows that when the same coupling method is used to obtain the hearing thresholds and RECD, equal and accurate estimates of real-ear sound pressure level are obtained.

Body site matters: an evaluation and application of a novel histological methodology on the quantification of mucous cells in the skin of Atlantic salmon, Salmo salar L

Mucous cell size and distribution were investigated in the skin of five salmon using a novel stereology-based methodology: one (48 cm) fish to test 15 tissue treatment combinations on measures of cell area and density on the dorsolateral region and, using the most suitable treatment, we mapped mucous cell differences between body regions on four (52 cm) salmon, comprising a male and a female on each of two diets. The section site, decalcification, embedding medium and plane of sectioning all impacted significantly on mucous cell size, whereas mucous cell density is more robust. There were highly significant differences in both mucosal density and mean mucous cell size depending on body site: the dorsolateral skin of the four salmon had significantly denser (about 8% of skin area) and larger (mean about 160 μm(2)) mucous cells, whereas the lowest mean density (about 4%) and smallest mean area (115 μm(2)) were found on the head. We found that 100 random measurements may be sufficient to distinguish differences >7 μm(2) in mean mucous cell areas. The results further suggest that salmon exhibit a dynamic repeatable pattern of mucous cell development influenced by sex, diet and possibly strain and season.

Age-related benefits of digital noise reduction for short-term word learning in children with hearing loss

PURPOSE

To determine the rate of word learning for children with hearing loss (HL) in quiet and in noise compared to normal-hearing (NH) peers. The effects of digital noise reduction (DNR) were examined for children with HL.

METHOD

Forty-one children with NH and 26 children with HL were grouped by age (8-9 years and 11-12 years). The children learned novel words associated with novel objects through a process of trial and error. Functions relating performance across trials were calculated for each child in each listening condition and were compared.

RESULTS

Significant effects were observed for age (older > younger) in the children with NH and listening condition (quiet > noise) in the children with HL. Significant effects of hearing status were also observed across groups (NH > HL), indicating that the children with HL required more trials to learn the new words. However, word learning improved significantly in noise with the use of DNR for the older but not for the younger children with HL. Hearing aid history and signal-to-noise ratio did not contribute to performance.

CONCLUSION

Word learning was significantly reduced in younger children, in noise, and in the presence of hearing loss. Age-related benefits of DNR were apparent for children over 10 years of age.

Children's performance in complex listening conditions: effects of hearing loss and digital noise reduction

PURPOSE

To determine the effect of hearing loss (HL) on children's performance for an auditory task under demanding listening conditions and to determine the effect of digital noise reduction (DNR) on that performance.

METHOD

Fifty children with normal hearing (NH) and 30 children with HL (8-12 years of age) categorized words in the presence of auditory or visual competitors, or both. Stimuli were presented at 50 dB SPL at a 0-dB signal-to-noise ratio. Children with HL were fitted with behind-the-ear hearing aids that had DNR technology. When DNR was activated, output decreased 4 dB, and signal-to-noise ratio increased 2 dB.

RESULTS

Significant main effects of group and age were observed. Performance for both groups decreased in noise, and the performance of the children with HL decreased further with the addition of the visual task. However, performance was unaffected by DNR. For the children with HL, stimulus audibility and communication skills contributed significantly to performance, whereas their history of hearing aid use did not.

CONCLUSIONS

For the children with HL, tasks unrelated to hearing interfered with their ability to participate in the auditory task. Consistent with previous studies, performance in noise was unaffected by DNR.

Immediate and long-term effects of hearing loss on the speech perception of children

The purpose of the present study was to examine the immediate and long-term effects of hearing loss on the speech perception of children. Hearing loss was simulated in normally-hearing children and their performance was compared to that of children with hearing loss (long-term effects) as well as to their own performance in quiet (immediate effects). Eleven children with normal hearing (7-10 years) were matched to five children with mild to moderate sensorineural hearing loss (8-10 years). Frequency-shaped broadband noise was used to elevate the hearing thresholds of the children with normal hearing to those of their matched hearing-impaired peer. Meaningful and nonsense sentences were presented at five levels and quantified using an audibility index (AI). Comparison of the AI functions calculated for each group and listening condition revealed a significant, immediate effect of elevated hearing thresholds in the children with normal hearing but no long-term effects of hearing loss. The results of this study suggest that hearing loss affects speech perception adversely and that amplification does not fully compensate for those effects. However, the data suggest that over the long term children may develop compensatory strategies to reduce the effects of hearing loss.

Use of forward pressure level to minimize the influence of acoustic standing waves during probe-microphone hearing-aid verification

Probe-microphone measurements are a reliable method of verifying hearing-aid sound pressure level (SPL) in the ear canal for frequencies between 0.25 and 4 kHz. However, standing waves in the ear canal reduce the accuracy of these measurements above 4 kHz. Recent data suggest that speech information at frequencies up to 10 kHz may enhance speech perception, particularly for children. Incident and reflected components of a stimulus in the ear canal can be separated, allowing the use of forward (incident) pressure as a measure of stimulus level. Two experiments were conducted to determine if hearing-aid output in forward pressure provides valid estimates of in-situ sound level in the ear canal. In experiment 1, SPL measurements were obtained at the tympanic membrane and the medial end of an earmold in ten adults. While within-subject test-retest reliability was acceptable, measures near the tympanic membrane reduced the influence of standing waves for two of the ten participants. In experiment 2, forward pressure measurements were found to be unaffected by standing waves in the ear canal for frequencies up to 10 kHz. Implications for clinical assessment of amplification are discussed.

Effect of minimal hearing loss on children's ability to multitask in quiet and in noise

PURPOSE

The purpose of the present study was to examine the effect of minimal hearing loss (HL) on children's ability to perform simultaneous tasks in quiet and in noise.

METHOD

Ten children with minimal HL and 11 children with normal hearing (NH) participated. Both groups ranged in age from 8 to 12 years. The children categorized common words (primary task) while completing dot-to-dot games (secondary task) in quiet as well as in noise presented at 0 dB and +6 dB signal-to-noise ratios (SNRs). It was hypothesized that the children's progression through the dot-to-dot games would slow as they encountered more difficult listening environments. This hypothesis was based on the theory that listeners have limited cognitive resources to allocate to any combination of tasks.

RESULTS

The dot rate of both groups decreased similarly in the multitasking conditions relative to baseline. However, no other differences between groups or listening conditions were revealed. Significantly poorer word categorization was observed for the children with minimal HL in noise.

CONCLUSION

These data suggest that children with minimal HL may be unable to respond to a difficult listening task by drawing resources from other tasks to compensate.

Perceptual coherence in listeners having longstanding childhood hearing losses, listeners with adult-onset hearing losses, and listeners with normal hearing

Perceptual coherence, the process by which the individual elements of complex sounds are bound together, was examined in adult listeners with longstanding childhood hearing losses, listeners with adult-onset hearing losses, and listeners with normal hearing. It was hypothesized that perceptual coherence would vary in strength between the groups due to their substantial differences in hearing history. Bisyllabic words produced by three talkers as well as comodulated three-tone complexes served as stimuli. In the first task, the second formant of each word was isolated and presented for recognition. In the second task, an isolated formant was paired with an intact word and listeners indicated whether or not the isolated second formant was a component of the intact word. In the third task, the middle component of the three-tone complex was presented in the same manner. For the speech stimuli, results indicate normal perceptual coherence in the listeners with adult-onset hearing loss but significantly weaker coherence in the listeners with childhood hearing losses. No differences were observed across groups for the nonspeech stimuli. These results suggest that perceptual coherence is relatively unaffected by hearing loss acquired during adulthood but appears to be impaired when hearing loss is present in early childhood.

Vocalizations of infants with hearing loss compared with infants with normal hearing: Part I--phonetic development

OBJECTIVE

Infants with hearing loss are known to be slower to develop spoken vocabulary than peers with normal hearing. Previous research demonstrates that they differ from normal-hearing children in several aspects of prelinguistic vocal development. Less is known about the vocalizations of early-identified infants with access to current hearing technologies. This longitudinal study documents changes in prelinguistic vocalizations in early-identified infants with varying degrees of hearing loss, compared with a group of infants with normal hearing. It was hypothesized that infants with hearing loss would demonstrate phonetic delays and that selected aspects of phonetic learning may be differentially affected by restricted auditory access.

DESIGN

The vocalizations and early verbalizations of 21 infants with normal hearing and 12 early-identified infants with hearing loss were compared over a period of 14 mo (from 10 to 24 mo of age). Thirty-minute mother-child interaction sessions were video recorded at 6- to 8-wk intervals in a laboratory playroom setting. Syllable complexity changes and consonantal development were quantified from vocalizations and early verbalizations. Early behaviors were related to speech production measures at 36 mo of age. Participants with hearing loss were recruited from local audiology clinics and early intervention programs. Participants with normal hearing were recruited through day care centers and pediatrician offices.

RESULTS

Relative to age-matched, normal-hearing peers, children with hearing loss were delayed in the onset of consistent canonical babble. However, certain children with moderately-severe losses babbled on time, and infants with cochlear implants babbled within 2 to 6 mo of implantation. The infants with hearing loss had smaller consonantal inventories and were slower to increase syllable shape complexity than age-matched normal-hearing peers. The overall pattern of results suggested that consonant development in infants with hearing loss was delayed but not qualitatively different from children with normal hearing. Delays appeared to be less pronounced than suggested by previous research. However, fricative/affricate development progressed slowly in infants with hearing loss and divergence from the patterns of normal-hearing children was observed. Six children (2 with normal hearing; 4 with hearing loss) were identified as atypical, based on their rates of development. At 24 mo of age, these children persisted in producing a high proportion (0.59) of vocalizations lacking consonants, which was negatively correlated with Goldman-Fristoe scores at 36 mo (r = -0.60).

CONCLUSIONS

Results suggest that early-identified children are delayed in consonant and syllable structure development, which may influence early word learning rates. Fricative/affricate development appears to be challenging for some infants with hearing loss. This may be related to the effects of sensorineural hearing loss on high-frequency information, restricted bandwidth provided by amplification, and reduced audibility in contexts of noise and reverberation. Delayed fricative use may have implications for morphological development. Atypically slow rates of change in syllable development may indicate that a child is at risk for delayed speech development.

Vocalizations of Infants with Hearing Loss Compared with Infants with Normal Hearing: Part II – Transition to Words

OBJECTIVE

By 24 mo of age, most typically developing infants with normal hearing successfully transition to the production of words that can be understood about 50% of the time. This study compares early phonological development in children with and without hearing loss to gain a clearer understanding of the effects of hearing loss in early-identified children. A secondary goal was to identify measures of early phonetic development that are predictors of later speech production outcomes.

DESIGN

The vocalizations and early words of 21 infants with normal hearing and 12 early-identified infants with hearing loss were followed longitudinally over a period of 14 mo (from 10 to 24 mo of age). Thirty-minute mother-child interaction samples were video recorded at 6- to 8-wk intervals in a laboratory playroom setting. Vocalizations produced at 16 and 24 mo were categorized according to communicative intent and recognizable words versus other types. Groups were compared on the structural complexity of words produced at 24 mo of age. Parent report measures of vocabulary development were collected from 10 to 30 mo of age, and Goldman-Fristoe Test of Articulation scores at 36 mo were used in regression analyses.

RESULTS

Both groups increased the purposeful use of voice between 16 and 24 mo of age. However, at 24 mo of age, the toddlers with hearing loss produced significantly fewer words that could be recognized by their mothers. Their samples were dominated by unintelligible communicative attempts at this age. In contrast, the samples from normal hearing children were dominated by words and phrases. At 24 mo of age, toddlers with normal hearing were more advanced than those with hearing loss on seven measures of the structural complexity of words. The children with normal hearing attempted more complex words and productions were more accurate than those of children with hearing loss. At 10 to 16 mo of age, the groups did not differ significantly on parent-report measures of receptive vocabulary. However, the hearing loss group was much slower to develop expressive vocabulary and demonstrated larger individual differences than the normal hearing group. Six children identified as atypical differed from all other children in vowel accuracy and complexity of word attempts. However, both atypical infants and typical infants with hearing loss were significantly less accurate than normal hearing infants in consonant and word production. Early measures of syllable production predicted unique variance in later speech production and vocabulary outcomes.

CONCLUSIONS

The transition from babble to words in infants with hearing loss appears to be delayed but parallel to that of infants with normal hearing. These delays appear to exert significant influences on expressive vocabulary development. Parents may appreciate knowing that some children with hearing loss may develop early vocabulary at a slower rate than children with normal hearing. Clinicians should monitor landmarks from babble onset through transitions to words. Indicators of atypical development were delayed and/or limited use of syllables with consonants, vowel errors and limited production of recognizable words.

Evidence suggesting that Homo neanderthalensis contributed the H2 MAPT haplotype to Homo sapiens

The tau (MAPT) locus exists as two distinct clades, H1 and H2. The H1 clade has a normal linkage disequilibrium structure and is the only haplotype found in all populations except those derived from Caucasians. The H2 haplotype is the minor haplotype in Caucasian populations and is not found in other populations. It shows no recombination over a region of 2 Mb with the more common H1 haplotype. The distribution of the haplotype and analysis of the slippage of dinucleotide repeat markers within the haplotype suggest that it entered Homo sapiens populations between approx. 10000 and 30000 years ago. However, sequence comparison of the H2 haplotype with the H1 haplotype and with the chimp sequence suggests that the common founder of the H1 and H2 haplotypes was far earlier than this. We suggest that the H2 haplotype is derived from Homo neanderthalensis and entered H. sapiens populations during the co-existence of these species in Europe from approx. 45000 to 18000 years ago and that the H2 haplotype has been under selection pressure since that time, possibly because of the role of this H1 haplotype in neurodegenerative disease.

Strong association of the Saitohin gene Q7 variant with progressive supranuclear palsy

Recent reports are inconclusive in showing that the Q7R polymorphism of the novel Saitohin gene, nested in intron 9 of the tau gene, is associated with AD. The authors show that this polymorphism is in complete linkage disequilibrium with the extended tau H1/H2 haplotype and that the Q variant and QQ genotype of Q7R are strongly associated with progressive supranuclear palsy, implicating it as a possibly important pathogenic candidate.

Tetrazolium-based assays for cellular viability: a critical examination of selected parameters affecting formazan production

The hydrogen acceptor 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) is commonly utilized to estimate cellular viability in drug screening protocols. The present investigation was prompted, in part, by observations that reduction of MTT to its colored reaction product, MTT formazan, varied between cell lines and with culture age. A correlation was established between the D-glucose concentration of the culture medium at the time of assay and the production of MTT formazan for cell lines representing seven tumor histologies. A decrease in the concentration of D-glucose from culture medium was accompanied by a decrease in MTT specific activity (MTT formazan/microgram cell protein) for a number of cell lines. Cells which extensively metabolized D-glucose exhibited the greatest reduction in MTT specific activity. Further evidence that the D-glucose concentration of the culture medium played an important role in MTT reduction was provided by experiments which demonstrated that transfer of cells to a glucose-free medium (L-15) was accompanied by an immediate decrease in MTT reduction which was pH independent. These studies suggested that cellular transport and constant metabolism of glucose were required for maximum MTT reduction. Decreases in the cellular concentration of the reduced pyridine nucleotides NADH and NADPH were accompanied by concomitant decreases in MTT formazan production. MTT formazan varied significantly among cell lines in both the kinetics of its formation and the degree of saturability exhibited. Apparent IC50 values for Adriamycin varied, in a cell line-specific manner, with MTT exposure time. These results indicate that MTT specific activity is significantly influenced by a number of parameters and suggest that assay conditions should be established which minimize their effects.