Effects of acute and chronic administration of mitomycin C on the induction of sister chromatid exchanges in vivo

Sister-chromatid exchange: second report of the Gene-Tox Program

This paper reviews the ability of a number of chemicals to induce sister-chromatid exchanges (SCEs). The SCE data for animal cells in vivo and in vitro, and human cells in vitro are presented in 6 tables according to their relative effectiveness. A seventh table summarizes what is known about the effects of specific chemicals on SCEs for humans exposed in vivo. The data support the concept that SCEs provide a useful indication of exposure, although the mechanism and biological significance of SCE formation still remain to be elucidated.

Persistence of chromosomal lesions induced in actively proliferating bone marrow cells of the mouse

The persistence of chromosomal lesions induced in vivo by mitomycin C (MMC) was evaluated by cytogenetic analysis of mouse bone marrow cells. Chromosome aberration (CA) and micronucleus (MN) frequencies were estimated at different times after treatment, up to 42 days. The frequency of CA per cell decreased in the first 3 days after treatment, but a secondary peak appeared on the 4th day, followed by a stabilization around 0.03 CA per cell (significantly different from the control value), which persisted up to 17 days. At the next time intervals tested (28 and 42 days), the CA frequency returned to the control level. In disagreement with these data obtained directly on metaphases, the MN frequency, as evaluated in polychromatic erythrocytes, decreased quickly after treatment, reaching the control value on the 5th day. We attempted to enhance the sensitivity of the MN test by using CREST antibodies and indirect immunofluorescence. However, higher proportions of CREST- MN in treated than in control animals were observed only at short time intervals, confirming the results obtained with the conventional MN assay.

Temporal SCE and cytotoxicity responses of murine cells following in vivo treatment with MNU or L-PAM

Time-dependent SCE responses of bone marrow and cultured spleen lymphocytes of BDF1 mice to in vivo treatment with MNU or L-PAM were studied. L-PAM was generally more active than MNU in producing elevated SCEs. Increases of 4-5 times control levels were produced in lymphocytes cultured at 1 or 24 h after i.p. injection of 4.95 mumoles/kg of L-PAM whereas an approximately 3-fold increase was produced by an acute injection of 190 mumoles/kg of MNU. Temporal SCE responses of bone marrow cells were carried out with doses of L-PAM (4.95 mumoles/kg) and MNU (131 mumoles/kg) found to be noncytotoxic by analysis of relative percentages of first, second, and third generation cells. The SCE response of second generation bone marrow cells (greater than 7 time baseline) to MNU was maximum when treatment was at the first cycle and decreased rapidly with increasing time prior to, or after the start of BrdUrd infusion. By contrast nonreciprocal (NR) SCE responses of third generation progeny never exceeded a 2-fold increase over baseline. Dramatic inhibition of cell cycling by MNU was evident as the reciprocal (R) SCEs in third generation cells increased, and exceeded NR SCEs, with increased treatment time intervals after the start of BrdUrd infusion. Similar dramatic cytotoxicity of L-PAM was apparent in time-dependent SCE response studies. An increased BrdUrd infusion time (28 h rather than the usual 26.5 h) was necessary to achieve adequate numbers of third division cells. Maximum SCE responses of the latter cells to L-PAM did not exceed 3 times baseline levels, whereas maximum responses of greater than 9 times control levels were produced in second generation cells. Comparison of SCE responses of second and third generation progeny of similarly treated cell populations, appears to provide a more sensitive assessment of cytotoxicity than does the conventional method of cell cycle analysis.

Repair of sister-chromatid exchange-inducing lesions in mutagen-treated cultures of human whole blood and purified fresh or frozen lymphocytes

Repair of mutagen-induced lesions that result in sister-chromatid exchanges was evaluated in 10 normal individuals. The mutagens used were mitomycin C (MMC), 4-nitroquinoline 1-oxide (4NQO), and N-methyl-N'nitro-N-nitrosoguanidine (MNNG). Cultures of whole blood, freshly purified lymphocytes, or purified lymphocytes cryopreserved for 6 months were analyzed after the mutagen treatments. All 3 mutagens induced reparable damage as evaluated by comparison of sister-chromatid exchanges between cultures that were given time to repair induced damage before 5-bromo-2'-deoxyuridine (BrdUrd) was added to the culture medium with those where BrdUrd was added immediately after the administration of the mutagens (MMC or 4NQO) or at culture initiation (MNNG). Repair of mutagen-induced DNA damage was detected in all 3 culture types; thus cryopreservation did not appear to alter the capacity of lymphocytes to repair mutagen-induced lesions. Quantitative differences in apparent repair capabilities were observed among individuals. Variability also existed among the different culture types within an individual, suggesting that caution should be exercised in interpreting these apparent differences.

In vivo and in vivo/in vitro kinetics of cyclophosphamide-induced sister-chromatid exchanges in mouse bone marrow and spleen cells

In several acute and chronic exposures to various chemicals in vivo and in vitro, the average sister-chromatid exchange (SCE) frequencies in human, mouse, rat, and rabbit lymphocytes generally decrease with time following treatment. The rate of this decline varies, but little data have been published pertaining to the comparative kinetics of SCEs both in vivo and in vivo/in vitro (exposure of animals to the test compound and culturing of cells) simultaneously in the same tissues. In this study, a single dose of cyclophosphamide (40 mg/kg) was injected for varying periods (6-48 h) and its effects, as assessed by the induction of SCEs, were analyzed under both in vivo and in vivo/in vitro conditions in mouse bone marrow and spleen cells. In vivo, the cyclophosphamide-induced SCEs increased with increasing time up to 12 h, stayed at approximately the same level until 24 h, and then decreased with increase in post-exposure time. However, the SCE levels remained significantly higher than controls at 48 h post-exposure time in both bone marrow and spleen cells. Under in vivo/in vitro conditions, the SCEs in bone marrow decreased with increase in post-exposure time until reaching control values by 48 h post exposure. However, in spleen cells, the decrease in SCE level was gradual, and by 48 h post-exposure time, the cells still had approximately 6 times higher SCEs than the control values. These results suggest that there are pharmacokinetic differences for cyclophosphamide in mouse bone marrow and spleen. Also, there is a differential SCE response to cyclophosphamide under in vivo and in vivo/in vitro conditions.

Effects of antimutagenic flavourings on SCEs induced by chemical mutagens in cultured Chinese hamster cells

Effects of antimutagenic flavourings such as vanillin, ethylvanillin, anisaldehyde, cinnamaldehyde, coumarin and umbelliferone on the induction of SCEs by MMC were investigated in cultured Chinese hamster ovary cells. None of these 6 flavourings showed any SCE-inducing activity by themselves. However, an obvious increase in the frequencies of SCEs was observed when MMC-pretreated cells were cultured in the presence of each flavouring. All these compounds have either an alpha, beta-unsaturated carbonyl group or a carbonyl functionality neighbouring the phenyl group which may react with an enzyme SH-group and cause higher-order structure changes. SCE-enhancing effects of vanillin were further investigated on 6 other kinds of mutagens. Vanillin was also effective on SCEs induced by EMS, ENNG, ENU or MNU. On the other hand, MMS- or MNNG-induced SCEs were not influenced at all by vanillin. SCE-enhancing effects of vanillin seemed to be dependent on the quality of lesions in DNA.

The effect of agent treatment time on the induction of sister-chromatid exchanges in mouse bone marrow cells in vivo

The effect of time of agent administration, via intraperitoneal injection, on the yield of SCEs in bone marrow cells of male B6C3F1 mice was determined for cyclophosphamide (CP), 7,12-dimethylbenz[a]anthracene (DMBA) and mitomycin C (MMC). Animals were treated with several doses of each carcinogen/mutagen at 3 different treatment times: -1, +1 and +8 h in relation to the onset of BrdUrd administration. The results of these studies indicate that the optimal treatment time for inducing a maximal SCE response is agent-specific. For CP, the slope of the SCE response was greatest at the +8 h treatment time while the maximal response for DMBA occurred at the -1 h treatment time. For MMC, the slope of the SCE response was independent of treatment time and of the method of bromodeoxyuridine administration (intravenous infusion vs. tablet implantation) but dependent on the laboratory conducting the study (Brookhaven National Laboratory vs. Oak Ridge Associated Universities). Based on the results of these studies, the +1 h acute treatment time is considered optimal for the in vivo cytogenetic evaluation of suspect chemicals for genotoxic activity when bone marrow is used as the target cell population.

Proliferation-dependent reduction of sister-chromatid exchange frequency induced by mitomycin C in human lymphoblastoid cells and its suppression by inhibitors of DNA replication

A high frequency of sister-chromatid exchange (SCE) induced in cells of a human lymphoblastoid cell line, NL3, by 2-h treatment with 1 microM mitomycin C (MMC) was maintained after holding the treated cells in a nonproliferating state for 48 h before cells were transferred into the BrdUrd-containing medium for SCE assay. The same was observed in cells treated with 4-nitroquinoline 1-oxide (4NQO) or ethyl methanesulfonate (EMS). In contrast, when MMC-treated cells were transferred into a growth medium and allowed to proliferate for various periods of time before SCE assay, MMC-induced SCE frequency decreased with time and reached near control level after 48 h. The reduction in SCE was also observed in 4NQO-treated cells, though to a lesser extent, but not in EMS-treated cells. When hydroxyurea or 1-beta-D-arabinofuranosylcytosine was given as a post-MMC treatment during this recovery process, such a reduction of SCE frequency was suppressed and the extent of the suppression appears to be roughly parallel to their ability to inhibit DNA replication. Cycloheximide and 5-azacytidine also exerted a similar inhibitory effect on the reduction of SCE. Benzamide and caffeine had no appreciable effect. Our results indicate that the SCE-forming lesions induced by MMC can be eliminated only in proliferating cells, probably during DNA replication.

Induction of sister chromatid exchanges in human and rat hepatoma cell lines by cyclophosphamide and phosphoramide mustard and the effects of cytochrome P-450 inhibitors

The activity of the cytochrome P-450-associated metabolic pathway in human (HepG2) and rat (H4-II-E) hepatoma cells was examined. The genotoxic activities of cyclophosphamide and its direct acting metabolite, phosphoramide mustard, were studied in the hepatoma cells as cyclophosphamide is known to be metabolized by phenobarbital-inducible cytochrome P-450-associated metabolic activity. HepG2 and H4-II-E demonstrated the capacity to activate cyclophosphamide to forms capable of inducing sister chromatid exchanges in concentration-dependent fashion. Phosphoramide mustard induced a similar pattern of sister chromatid exchanges at concentrations three orders of magnitude lower than cyclophosphamide. The cytochrome P-450-associated enzyme inhibitors, SKF-525A and metyrapone, were found to reduce the level of cyclophosphamide-induced sister chromatid exchanges in HepG2 and H4-II-E, suggesting that cyclophosphamide was activated by this pathway in both hepatoma lines. Direct evidence for the presence of mRNA transcript coding for a phenobarbital-inducible cytochrome P-450 was demonstrated in HepG2 cells by Northern blot analysis. Comparison of genotoxic responses in human and rat hepatoma cells may allow for an evaluation of responses by different species to potentially mutagenic chemicals.

The kinetics of in vivo sister chromatid exchange induction in mouse bone marrow cells by ethylnitrosourea and methylnitrosourea

Administration of ethylnitrosourea (ENU) (20, 25, 40, 50, and 60 mg/kg body weight) or methylnitrosourea (MNU) (25, 40, 50, 60, 75, and 80 mg/kg body weight) to male CD-1 mice 2 hr after sc implantation of a 5-bromo-2'-deoxyuridine (BrdUrd) pellet (55 mg) resulted in a dose-dependent increase in sister chromatid exchanges (SCE) in bone marrow cells. Treatment with ENU (50 mg/kg body weight) at several time points prior to BrdUrd implantation resulted in a multiphasic curve of SCE induction indicating at least two events that result in SCEs. Treatment with ENU at the time of BrdUrd implantation and post-BrdUrd reflected a similar mechanism apparent in the pre-BrdUrd curve. Treatment with MNU (50 mg/kg body weight) pre- and post-BrdUrd resulted in a linear monophasic curve of SCE induction in both the pre- and post-BrdUrd time periods. The overall MNU time-course curve resembled an inverted V function suggesting the mechanisms of SCE induction for ENU and MNU are different. These observations suggest that at least one explanation for the differences in the time courses for ENU and MNU SCE induction may result from a more persistent lesion being induced by ENU. In addition, these results indicate that in vivo SCE protocols which utilize a single acute chemical exposure at or near the time of BrdUrd labeling may not be useful for judging the relative activities of genotoxic agents.

Dose-responsive increase in sister-chromatid exchanges in bone-marrow cells of mice exposed nose-only to whole cigarette smoke

The effect of whole cigarette smoke exposure on bone-marrow sister-chromatid exchanges (SCEs) was studied in B6C3F1 mice. Animals were exposed nose-only to 10% (v/v) cigarette smoke 5 days/week for 2 weeks. Four dose levels of cigarette smoke (1, 4, 9 and 18 exposures/day) were studied using 2 cigarette types, Kentucky reference 3A1 (3A1) and American Blend (AB). A single exposure represented approximately 1 cigarette. A dose-dependent increase in SCEs was observed for both the 3A1 and AB cigarettes at dose levels which had no effect on bone-marrow cell-replication kinetics. These findings represent the first demonstration of a dose-responsive increase in cigarette smoke-induced SCEs in a rodent model system.

Kinetics of sister-chromatid exchange induction by different carcinogens in C57BL/6J and DBA/2 mice

Sister-chromatid exchange (SCE) levels were determined in bone marrow cells of DBA/2 and C57BL/6J mice at 18, 24, 30 and 48 h after treatment with 10 mg/kg cyclophosphamide (CP), 300 mg/kg urethane or 25 mg/kg N-nitroso-N-methylurea (NMU). DBA/2 mice showed higher SCE frequencies than C57BL/6J mice at all time points examined after CP and urethane treatment, whereas NMU administration resulted in similar SCE levels in both strains. After CP and urethane treatments, SCE values reached the highest level at 18 h, were similarly high at 24 and 30 h, and returned to base-line level at 48 h. In NMU-treated mice, the SCE values had the same level at 18, 24 and 30 h and returned to normal at 48 h. The results are interpreted as indicating that different metabolic capabilities are responsible for the difference in SCE response in the two strains.

Genotoxicity of garlic, turmeric and asafoetida in mice

The genotoxic effects of orally administered garlic and turmeric were evaluated in bone-marrow cells of mice by performing the micronucleus test. Another spice, asafoetida, was tested for the induction of sister-chromatid exchanges (SCEs) in spermatogonia of mice. Results of the micronucleus test with garlic and turmeric were not significantly different from control values. Orally administered asafoetida, however, showed a weak SCE-inducing effect in spermatogonia.

The effects of diagnostic ultrasound on sister chromatid exchange frequencies: a review of the recent literature

The precise nature and significance of SCEs are still not understood. It is clear that SCE analysis is a sensitive means of detecting genotoxic agents, and that SCEs are correlated with mutation, in vitro transformation, and cancer. SCEs, as all other short-term assays, are subject to false positives and false negatives. Therefore, it is possible that even if ultrasound induces SCE, the effects may not be deleterious, although the reverse could be true as well. The majority of studies have not observed SCE induction after exposure to diagnostic ultrasound, however two positive responses have been reported. Such discrepancies cannot be explained until investigators adopt a uniform protocol for ultrasound exposure. The currently widespread use of ultrasound in antepartum diagnosis, and the rate at which its use increases, argue strongly for additional research into its potential for genetic toxicity.

Persistence of SCE-inducing lesions after G0 exposure of human lymphocytes to differing classes of DNA-damaging chemicals

We conducted studies to determine whether cycling human lymphocytes are equally efficient in repairing sister chromatid exchange (SCE)-producing lesions induced by differing classes of DNA-damaging chemicals. Lymphocytes were pulse-treated during G0 with mitomycin C (MMC), N,N',N''-triethylenethiophosphoramide (ThioTEPA), ethylmethanesulfonate (EMS), or cis-diamminedichloroplatinum (cis-DDP). Bromodeoxyuridine (BrdUrd) was added to the 72 hr cultures at 0 hr or at 48 hr after phytohemmagglutinin stimulation. The concentrations of chemicals employed induced a greater than 2-fold increase in SCEs in second-division metaphases from lymphocytes cultured in the presence of BrdUrd for the entire 72 hr. The analysis of SCEs in uniformly harlequinized metaphases from G0-treated lymphocytes cultured in BrdUrd for the terminal 24 hr showed no increase above baseline after exposure to MMC, and intermediate increases above baseline after exposures to ThioTEPA and cis-DDP. However, after G0 treatment with EMS, the observed SCE frequency was consistent with that expected had all DNA lesions persisted and continued to give rise to SCEs during 3 cell cycles. These findings suggest that cycling human lymphocytes are not equally efficient in eliminating SCE-producing lesions after exposure to differing classes of DNA-damaging chemicals.

Induction and rapid repair of sister-chromatid exchanges in multiple murine tissues in vivo by diepoxybutane

Sister-chromatid exchange (SCE) induction by the direct-acting bifunctional carcinogen, diepoxybutane (DEB), was investigated in multiple tissues in vivo. The log-log dose SCE response relationship was found to be parallel to that previously reported for DEB induction of lung adenomas. However, the SCE assay is approximately 20 times as sensitive in detecting genotoxic effects of DEB than indicated by the lung adenoma assay. Examination of second and third division cells following various treatment protocols revealed that regardless of the nature of initially induced lesions, they are rapidly repaired with no evidence of persistence beyond 1 cell cycle.

Induction of sister-chromatid exchanges by chemotherapeutic drugs in spermatogonia of mice: effects of procarbazine, adriamycin, cyclophosphamide and mitomycin C

The chemotherapeutic drugs procarbazine (PCB), adriamycin (ADR), cyclophosphamide (CP) and mitomycin C (MMC) were evaluated in vivo for induction of sister-chromatid exchanges (SCEs) in differentiating spermatogonia of mice. There was a dose-dependent increase in SCE induced by the drugs. The lowest doses that enhanced the SCE rate to greater than twice baseline frequency were: 10 (PCB), 0.25 (ADR), 5 (CP) and 0.25 (MMC) mg/kg body weight. Based on the x-fold increase in SCE over baseline frequency, induced by the highest test dose (which permitted analysis of SCE), the drugs evaluated were ranked as follows: CP greater than MMC greater than PCB greater than ADR. The persistence of SCE-inducing lesions in spermatogonia was investigated by giving a treatment of CP (20 mg/kg body weight), 6 and 9 days before completion of 5-bromodeoxyuridine (BrdUrd) administration. Within 6-9 days, the SCE rate returned to the baseline level.

Sister-chromatid exchanges induced in swine lymphocytes by chronic oral doses of dimethylbenzanthracene

Sister-chromatid exchanges were scored at 3-week intervals in lymphocytes of female swine ingesting daily doses of 1.25 or 2.50 mg/kg of 7,12-dimethylbenzanthracene (DMBA) for 160 days. Exchanges increased with time for about 120 days then reached a plateau at approximately 2.5 times their pretreatment level. No increase in chromosome aberrations could be identified as resulting from ingestion of the chemical. Week-old progeny of animals that had ingested the chemical throughout pregnancy showed no increase of exchanges in their peripheral lymphocytes.

Short-term tests for transplacentally active carcinogens. A comparison of sister-chromatid exchange and the micronucleus test in mouse foetal liver erythroblasts

The effectiveness of 6 chemicals (benzo[a]pyrene, (BaP), cyclophosphamide (CP), diethylnitrosamine (DEN), methyl methanesulphonate (MMS), mitomycin C (MC) and procarbazine (PC) ) as inducers of micronuclei in foetal liver and maternal bone marrow erythroblasts has been determined, and related to that of gamma-radiation. CP, DEN, MMS and PC were all more effective in the foetal liver. The induction of micronuclei and SCEs by each chemical in foetal erythroblasts after in vivo exposure was measured. When expressed as induction of sister-chromatid exchanges (SCEs) per erythroblast/induction of micronuclei per erythroblast (/microM/kg), the ratios obtained were MC 580, BaP 470, DEN 430, CP 258, MMS 140 and PC 13. The lowest doses detected as potentially genotoxic by each test in foetal liver erythroblasts are (with the exception of PC which is a relatively ineffective inducer of SCEs) similar. When isolated foetal livers were exposed in vitro, SCE dose responses to BaP, MC, MMS and PC could be directly related to those from in vivo exposure, indicating the role of the foetal liver in metabolic activation, but CP was considerably more cytotoxic. The transplacental micronucleus test, and in vivo/in vitro method for SCEs in foetal liver erythroblasts, provide sensitive, complementary assays for genotoxic effects of chemicals during prenatal life. Since foetal liver possesses greater metabolic potential than adult bone marrow, the transplacental tests respond to genotoxic agents not detected by bone-marrow systems.

The kinetics of in vivo sister chromatid exchange induction in mouse bone marrow cells by alkylating agents: cyclophosphamide

Administration of cyclophosphamide (5, 10, 20 and 25 mg/kg body weight) to male CD-1 mice 2 hr after subcutaneous implantation of a 5-bromo-2'-deoxyuridine (BrdUrd) pellet (55 mg) resulted in a dose-dependent increase in sister chromatid exchanges (SCE) in bone marrow cells. Treatment with cyclophosphamide (15 mg/kg body weight) at the time of BrdUrd implantation and 2, 6.5, and 13 hr post-BrdUrd implantation resulted in the induction of approximately 19 SCE/cell indicating that the bone marrow SCE response was independent of the time of administration. Treatment with cyclophosphamide (15 mg/kg body weight) at 26, 19, 13, and 6 hr prior to BrdUrd implantation resulted in baseline SCE (3.3 SCE/cell) at 26 hr with an increasing number of SCE/cell with decreasing time prior to BrdUrd implantation. These results compare favorably with those obtained by Kram et al [1981] with mitomycin C (MMC) using a similar protocol. The time-dependent induction of SCE is qualitatively similar for CP and MMC, both of which are bifunctional alkylating agents metabolically activated by oxidation and reduction, respectively, and suggests that these two compounds may induce SCE by a similar mechanism.

SCE evaluations in human lymphocytes after G0 exposure to mitomycin C. Lack of expression of MMC-induced SCEs in cells that have undergone greater than two in vitro divisions

We conducted a series of experiments designed to determine whether DNA damage induced in G0 lymphocytes by mitomycin C (MMC) would be expressed as sister-chromatid exchanges during the second and third post-treatment cell cycles. Lymphocytes from normal donors were exposed to MMC for 2 h prior to culture in the presence of phytohemagglutinin. MMC-treated and control cells were subsequently exposed to bromodeoxyuridine (BrdUrd) for the entire culture period (i.e. 48 h or 72 h) or for the terminal 24 h of 72-h cultures. We observed a 3-4-fold increase in SCEs in MII metaphases from lymphocytes treated with MMC and cultured in the presence of BrdUrd for the entire culture period. In contrast, in replicate cultures of MMC-treated lymphocytes that were exposed to BrdUrd for the terminal 24 h only, the SCE frequency in uniformly harlequinized metaphases was not significantly different from that observed in control cultures. We interpret these data as providing evidence that MMC-induced lesions (or alterations) in the DNA of G0 lymphocytes are probably expressed as SCEs during the first period of mitogen-induced DNA synthesis, and that these lesions do not persist and give rise to SCEs in subsequent cell divisions.

The longevity of chemically induced sister chromatid exchanges in Chinese hamster ovary cells

The persistence of the lesions leading to sister chromatid exchanges (SCEs) following acute exposure of Chinese hamster ovary (CHO) cells to direct-acting chemical mutagens was measured. The results revealed that these lesions (and consequently SCEs) are rapidly eliminated from cells. SCE levels fell to near control values by the third or fourth day (six and eight cell cycles, respectively) following exposure of CHO cells to quinacrine mustard (QM) and mitomycin C (MMC). In contrast, cells exposed to methyl methanesulfonate (MMS) showed a small but significant increase in SCE level over control up to and including the fifth day following exposure (approximately ten cell cycles), suggesting that the behavior of these lesions in cells is influenced, at least in part, by the mechanism by which a specific agent interacts with DNA. The possibility that the decline in SCE level was due to the loss of cell populations with high numbers of exchanges was eliminated by the assessment of cloning efficiencies of treated and control cultures.

Sister chromatid exchanges induced in rabbit lymphocytes by ethylene oxide after inhalation exposure

Ethylene oxide, which is the simplest epoxide and an extremely important commercial compound, has been used by many investigators as a model compound to study mutagenicity by alkylation of DNA. Knowledge of in vivo dose-effect relations under experimental conditions may provide further insight into the dynamics of the sister chromatid exchange (SCE) response. It may also provide information on temporal aspects of sampling design for human worker populations. Groups of four male New Zealand white rabbits were exposed in inhalation chambers to 0, 10, 50, and 250 parts per million (ppm) ethylene oxide for 6 hr a day, 5 days a week, for 12 weeks. Peripheral blood samples were taken before the start of exposure, at intervals during exposure, and up to 15 weeks after the end of exposure to measure SCE rates in peripheral lymphocytes as well as standard hematological endpoints. Additionally, the level of reduced glutathione (GSH) in liver and blood was measured in a set of concurrently exposed animals at the end of the 12-week exposure. Results show that exposure to 10 ppm does not cause a detectable increase in SCE rates. However, exposure to 50 and 250 ppm does cause an increase in SCEs that decreases after exposure ends, but still remains above baseline levels 15 weeks after exposure. Hematological and GSH measurements did not differ between control and exposed groups. These results indicate that inhalation exposure to the mutagenic alkylating agent ethylene oxide results in a dose-related SCE effect, and that SCE is a more sensitive indicator of exposure than either standard hematological end points or GSH levels.