Interactions between Sendai virus and human erythrocytes

Antiviral effect of copper chloride on feline calicivirus and synergy with ribavirin in vitro

Background

Feline calicivirus (FCV) is a common and highly prevalent pathogen causing upper respiratory diseases in kittens and felines in recent years. Due to the substantial genetic variability of the viral genes, existing vaccines cannot provide complete protection. Therefore, research on FCV antiviral drugs has received much attention.

Results

In this study, we found that copper chloride had dose-dependent antiviral effects on FCV in F81 cells. We also found that the combination of copper chloride and ribavirin had a synergistic protective effect against FCV in F81 cells. In contrast, the combination of copper chloride and horse anti-FCV immunoglobulin F (ab’)₂ showed an antagonistic effect, likely because copper chloride has an effect on F (ab’)₂ immunoglobulin; however, further research is needed to clarify this supposition.

Conclusions

In summary, we found that copper chloride had low cytotoxicity and significant antiviral effects on FCV in F81 cells, providing a new drug candidate for the prevention and treatment of FCV infection.

Differential Features of Fusion Activation within the Paramyxoviridae

Paramyxovirus (PMV) entry requires the coordinated action of two envelope glycoproteins, the receptor binding protein (RBP) and fusion protein (F). The sequence of events that occurs during the PMV entry process is tightly regulated. This regulation ensures entry will only initiate when the virion is in the vicinity of a target cell membrane. Here, we review recent structural and mechanistic studies to delineate the entry features that are shared and distinct amongst the Paramyxoviridae. In general, we observe overarching distinctions between the protein-using RBPs and the sialic acid- (SA-) using RBPs, including how their stalk domains differentially trigger F. Moreover, through sequence comparisons, we identify greater structural and functional conservation amongst the PMV fusion proteins, as compared to the RBPs. When examining the relative contributions to sequence conservation of the globular head versus stalk domains of the RBP, we observe that, for the protein-using PMVs, the stalk domains exhibit higher conservation and find the opposite trend is true for SA-using PMVs. A better understanding of conserved and distinct features that govern the entry of protein-using versus SA-using PMVs will inform the rational design of broader spectrum therapeutics that impede this process.

Inactivation and morphological changes of avian influenza virus by copper ions

The infectivity of the H9N2 virus to MDCK cells was time-dependently inhibited by Cu(2+) at concentrations of 2.5-250 microM. In 25 microM Cu(2+) solution, the virus titer decreased by approximately 3 and 4 log within 3 and 6 h, respectively. Compared to Cu(2+), Zn(2+) was much less effective in virus inactivation. The H9N2 virus hemagglutinin activity was not affected by 2.5-250 microM Cu(2+). The H9N2 virus neuraminidase (NA) activity was drastically reduced by 25 mM Cu(2+), marginally reduced by 250 microM Cu(2+), and not affected by 25 microM Cu(2+). Thus, we found that copper ions suppress the infectivity of influenza virus at lower concentrations at which neither NA nor hemagglutination inhibition occurs. Electron microscopic analysis revealed morphological abnormalities of the Cu(2+)-treated H9N2 virus. Additional studies should be undertaken to clarify the mechanism underlying the antiviral effect of copper ions on influenza virus.

Chapter 9 Fusion of Viral Envelopes with Cellular Membranes

This chapter reviews some characteristic features of membrane fusion activity for each virus and discusses the mechanisms of membrane fusion, especially low pH-induced membrane fusion. It concentrates on the interaction of the hydrophobic segment with the target cell membrane lipid bilayer and suggests the entrance of the segment into the lipid bilayer hydrophobic core as a key step in fusion. The envelope is a lipid bilayer membrane with the virus specific glycoproteins spanning it. The bilayer originates from the host cell membrane and has a lipid composition and transbilayer distribution quite similar to the host's. The viral glycoproteins have the functions of binding to the target cell surface and fusion with the cell membranes. The two functions are carried by a single glycoprotein in influenza virus (HA), vesicular stomatitis virus (VSV) G glycoprotein, and Semliki Forest virus SFV E glycoprotein. In Sendai virus (HVJ), the functions are carried by separate glycoproteins, hemagglutinin-neuraminidase (HN) for binding and fusion glycoprotein (F) for fusion. When viruses encounter target cells, they first bind to the cell surface through an interaction of the viral glycoprotein with receptors.

Effect of anionic polymers on fusion of Sendai virus with human erythrocyte ghosts

The effect of anionic polymers (dextran sulfate, heparin and chondroitin sulfate) on fusion of Sendai virus with erythrocyte ghosts was studied. The effect of pH on the activity of these anionic polymers was also investigated. In order to examine the interaction of such polymers with the Sendai virion and erythrocyte ghost surfaces, the binding of virions to erythrocyte ghosts and the aggregation of virions and/or erythrocyte ghosts were also measured with respect to the same parameters. It was found that the anionic polymers suppressed the fusion of Sendai virus with erythrocyte ghosts. The order of effectiveness of the polymers in suppression was dextran sulfate greater than heparin greater than chondroitin sulfate, for the application of a same quantity (weight/ml) of the polymers. The lower the pH of the suspending medium, the more effective were the polymers in suppressing virion-erythrocyte ghost aggregation and fusion. The suppression of fusion was dependent on the concentration of the polymers applied: the higher the concentration of the polymer applied, the more the suppression was observed. Evidence from binding studies, turbidity measurements and electrophoretic mobility measurements indicates that the anionic polymers interact preferentially with the virion surface.

Pneumotropism of Sendai virus in relation to protease-mediated activation in mouse lungs

The pneumotropism of Sendai virus in mice was studied in relation to the activation and replication of the virus in the lung. Inactive Sendai virus grown in LLC-MK(2) cells, which possessed an uncleaved precursor glycoprotein, F, and was noninfectious to tissue culture cells, neither grew nor caused pathological changes in the lung of mice. When trypsin treatment was made which cleaved F into F(1) and F(2) subunits, the virus became activated so that it could initiate replication in the bronchial epithelium of the lung. In this case, the progeny virus was produced in the activated form and multiple-cycle replication occurred successively. A parallel relationship was found between the degree of the viral replication and that of clinical signs of the respiratory disease, body weight loss, and histopathological changes in the lung. A protease mutant, TR-2, which was able to be activated only by chymotrypsin but not by trypsin, could also initiate replication in the bronchial epithelium, when activated by chymotrypsin before inoculation into mice. The progeny virus, however, remained inactive, and the replication was limited to a single cycle, which resulted in the limited lung lesion. The overall results suggest that some activating mechanism for the progeny virus of wild-type Sendai virus exists in the lung of mice and the principle (activator) responsible for this phenomenon has a character similar to trypsin. The possible location of the activator is discussed.

Fusion of Sendai viruses or subviral envelope components with chicken erythrocytes observed by freeze-fracture electron microscopy

Four different types of envelope of Sendai virus or subviral components, that is, infectious and non-infectious virions, reassembled envelope particles (REP), and Tween-ether-treated envelope fragments (TE), were studied comparatively for membrane interactions with chicken erythrocytes by freeze-fracture electron microscopy, specifically for membrane alteration by envelope fusion. The freeze-fracture replicas of the attachment of the four envelopes in the cold exhibited a common pattern of impressions with attached envelopes, although the fracture plane traversed from erythrocyte to envelope at the periphery of the contact areas of three of the envelopes but not of TE, where the fracture plane mostly cut only through erythrocyte membranes impressed with TE. The freeze-fracture replicas of the four envelopes reacting with erythrocytes after a short incubation period at 37 C exhibited distinctive features: infectious virions and REP displayed evidence of envelope fusion, but non-infectious virions and TE showed a particular pattern of envelope association without fusion. Our data demonstrate that the pattern specific for envelope fusion is the formation of a continuous membrane from envelope to cell membrane in a cross fracture of an erythrocyte.

Enhancement of membrane-fusing activity of sendai virus by exposure of the virus to basic pH is correlated with a conformational change in the fusion protein

The effect of pH on the membrane-fusion activity of Sendai virus was examined (pH 5.0-9.5) by using, as assays of activity, hemolysis of chicken erythrocytes and the fusion of baby hamster kidney (BHK-21) cells. Exposure of virus to basic pH increased fusion activity; the optimum pH was found to be approximately equal to 9.0. All assays were carried out at pH 7.0, and the virus retained enhanced fusion activity after it was exposed to basic pH and returned to neutral pH. The enhanced fusion activity was correlated with an irreversible conformational change in the fusion protein (F protein) of the virus, as demonstrated by a change in the circular dichroism spectrum of the protein.

Interaction of cytomegalovirus immune complexes with host cells

After adsorption to host cells of a mixture of cytomegalovirus (CMV) and immunoglobulin G (IgG)-anti CMV, at least 99% of the surviving fraction consisted of infectious virus-antibody complexes which could be neutralized by anti-human IgG antibody. The virus-antibody complexes penetrated into cells and were uncoated more slowly than native virus. When the virus-antibody complexes were treated with anti-human IgG antibody before adsorption was allowed to take place, anti-human IgG antibody immune complexes were adsorbed to host cells less readily and uncoated less efficiently than were untreated complexes. Furthermore, after adsorption, anti-human IgG antibody-IgG-CMV complexes were further retarded in their penetration and uncoating. These observations suggest that neutralization of CMV by IgG-anti-CMV resulted from interference with the normal mechanisms of penetration and uncoating. Anti-human IgG antibody enhanced the degree of neutralization by augmenting the inhibitory effect of antibody on these stages of virus-host interaction and also reduced adsorption of the complex.

Immunoelectron microscopic study on interactions of noninfectious sendai virus and murine cells

The early interactions of LLC-MK2 cell-grown noninfectious Sendai virus and a murine cell line, P815 mastocytoma ascitic cells, were studied by electron microscopy, using the ferritin-conjugated antibody technique with anti-virus glycoprotein serum. For comparison, the interactions of egg-grown infectious Sendai virus with the same cells were also examined. When noninfectious virus was adsorbed to the cells in the cold, the cell membranes become partially invaginated at the site of contact of adsorbed virions, but ferritin-conjugated antibodies did not penetrate into the areas of envelope-cell membrane association. This pattern of virus attachment was similar to that of infectious virus attachment. Upon subsequent incubation at 37 degrees C, most of the adsorbed noninfectious virions were taken into cytoplasmic vesicles and then degraded, although a few virions remained attached to the cell membrane. No evidence of fusion of envelopes of noninfectious virions was obtained. On the other hand, envelopes of infectious virions fused with the cell membrane, and the transferred viral antigens diffused on the cell surfaces and then decreased in number.

Non-specific inhibition by virus particles of human lymphocytes mitogenesis

Many different types of virus particles including avian retroviruses, Friend leukaemia virus and Sendai virus are able, when coincubated with human peripheral blood lymphocytes in the presence of mitogens or alloantigens, to inhibit the usual proliferative responses that normally ensue. These effects are independent of infection and can be obtained using u.v.-inactivated viruses as well as virus-lymphocyte combinations which are non-physiological in nature. Lymphocytes which are preincubated with viruses for as little as 5 min, and then washed free of unbound virus, are significantly impaired in terms of ability to react to mitogenic stimulus. These events may be mediated, in part at least, by the virus-induced elaboration by mononuclear cells of a factor with lymphocyte inhibitory potential.

Influence of the age of sheep red blood cells on virus-induced and hypotonic hemolysis

The effect of red blood cell age on the susceptibility towards NDV- induced hemolysis has been investigated. It was found that young sheep red blood cells (SRBC), although more resistant to hypotonic hemolysis, are more susceptible towards viral hemolysis than older cells. This may be because young red blood cells have a higher content of neuraminic acid, which functions as a receptor for NDV.

Viral abrogation of lymphocyte mitogenesis: induction of a soluble factor inhibitory to cellular proliferation

PHA and Con A-driven mitogenesis of mouse C3H lymphocytes can be inhibited by co-incubation with a variety of different virus particles. These effects appear independent of infection, and can be obtained using UV-inactivated virus. Viruses may be added to spleen cell cultures as late as 46 h after co-incubation with mitogen, and still achieve significant inhibition of proliferative responsiveness. The described inhibition is apparently mediated, in part at least, by a soluble factor which is induced in splenic cultures following interaction with virus particles. This factor is apparently a product of macrophages. It does not posess interferon activity, but does have the ability to inhibit lectin- and alloantigen-driven mitogenesis, as measured in fresh cultures of splenic lymphocytes and in the mixed lymphocyte culture (MLC) reaction, respectively. Moreover, addition of virus to splenic cultures can apparently activate suppressor lymphocytes with the ability to inhibit proliferative responsiveness of fresh lymphocyte suspensions in the presence of Con A.

Nature of permeability changes in membrane of HeLa cells adsorbing Sendai virus

Adsorption of Sendai virus to HeLa cells induced in them an increased permeability to K+, Na+, Ca++, deoxyglucose, but not to fluorescein. The stimulation of uptake of 42K was temperature-dependent, did not occur below 15 degrees C, and was not inhibited by ouabain. The virus-induced increase in the uptake and release of 42K and of 3H deoxyglucose could not be mimicked by treatment of cells with linoleic acid, a procedure which increased the fluidity of the cellular membranes. The stimulatory effect of 0.5 mM ATP on the release of deoxyglucose was enhanced several fold in the presence of Sendai virus. These results seem to indicate the possible involvement of membranal enzymes such as e.g. protein kinase in the permeability changes induced by Sendai virus.

Inhibition by interferon of Sendai virus cytolysis

Treatment of HeLa cells with human interferons inhibited 51Cr release from cells induced by ultraviolet-inactivated Sendai virus. The inhibitory effect became apparent about 6 h after interferon treatment and persisted for 24 to 48 h. In the interferon-treated cells, the cytolysis was inhibited within 10 min after adding virus and the inhibitory action was suppressed by the treatment of the cells with cycloheximide. Mock interferon and mouse interferon did not inhibit the cytolysis and antiinterferon serum neutralized the effect of interferon. All these findings indicate that Sendai virus-induced cytolysis is inhibited by interferon per se. However, interferon did not have any influence on Sendai virus hemolysis.

Role of envelope proteins of paramyxoviruses in the modification of cell membrane antigens

Adsorption of paramyxoviruses to separated membranes of tumor cells produces neoantigens that are immunogenic in syngeneic mice (xenogenization). Thus it became possible to study this process by using modified virus or virus fractions. Membranes with adsorbed viral derivatives produced an immune response which was measured by cytotoxic and complement fixing antibodies and the appearance of autoimmune disease. The effect of viral preparations with reduced F (fusion) protein activity was compared to treatment of membranes with equivalent amounts of active or inactive virus as measured by hemagglutination. Viral preparations without hemolytic activity showed diminished adsorption to membranes and the immune response was reduced. Triton X 100 and desoxycholate extracted from membranes immunogenic material with associated paramyxovirus antigens.

Glycoproteins of Sendai virus are transmembrane proteins

Radiolabeled Sendai viral envelope proteins were incorporated into human erythrocyte membranes by the process of fusion of the viral envelope with the erythrocyte membrane. Inside-out (IO) vesicles were prepared from the erythrocyte membranes containing viral proteins, and the presence of viral proteins was assessed by electrophoresis in sodium dodecyl sulfate/polyacrylamide gels. Proteolysis with trypsin, chymotrypsin, or Pronase, which digests only the external surface of the IO vesicles (the cytoplasmic surface of the erythrocyte membrane) revealed that the viral nucleocapsid and the nonglycosylated inner-envelope (M) proteins were present on the external surface. In addition, small segments of the viral envelope glycoproteins (HN and F1) were removed by these proteases, while the major portions of the glycoproteins were protected from digestion, indicating that in the erythrocyte membrane they are transmembrane proteins. Results of experiments carried out on right side-out membranes, unsealed membranes, and membranes containing attached virus that was unable to fuse indicated that the results obtained with IO membranes could not be accounted for by contamination with these membrane species. The identify of the proteolysis products was confirmed by peptide mapping. The selective exposure of the cytoplasmic surface, and hence the internal components of the virus, in IO vesicles makes this membrane system an attractive model for studying the interactions involved in virus maturation at host cell membranes.

Non-specific effects of avian retrovirus co-incubation on lymphocyte function: abrogation of antigen- and mitogen-induced proliferative responsiveness

Peripheral blood lymphocytes from chickens bearing tumours induced by avian retroviruses can be stimulated to divide by group-specific antigens present in supernatant fluids of avian retrovirus-infected but not normal chicken embryo fibroblast (CEF) cells. Centrifugation studies revealed that the relevant antigenic activity is non-virion in nature. Indeed, the presence of avian retrovirus particles was found to be inhibitory to the capacity of sensitized lymphocytes to be stimulated in this antigen-driven blastogenesis assay. Similar results were obtained in lymphocyte mitogenesis experiments in which any of peripheral chicken lymphocytes or mouse splenic, lymph node or thymic lymphocytes were co-incubated with either concanavalin A or phytohaemagglutinin in the presence of numerous types of virus particles. This inhibitory effect was not due to infection of lymphocytes by the viruses tested, and was obtained in the case of lymphocyte-virus combinations for which the cells lacked the surface receptors required for viral entry. Virus could be added to lymphocyte cultures as late as 26 h after co-incubation with mitogen, and still inhibit the usual mitogenic response. In addition, co-addition of virus to lymphocytes in the presence of concanavalin A was found to block the capping of ligand-bound receptors which normally ensues. Pre-added virus did not, however, affect the ability of lectins to bind to cells.

Role of Ca++ in virus-induced membrane fusion. Ca++ accumulation and ultrastructural changes induced by Sendai virus in chicken erythrocytes

Some of the ultrastructural (freeze-etching technique), morphological, and biochemical effects of Sendai virus interaction with chicken erythrocytes have been studied under fusogenic (in the presence of CaCl2) and nonfusogenic (in the presence of ethyleneglycol-bis-N,N'-tetraacetic acid, [EGTA]) conditions. The following phenomena occur, irrespective of the presence of CaCl2 or EGTA: (a) binding of iodinated virus particles to chicken erythrocytes at 4 degrees C and their partial release from the cells at 37 degrees C; (b) gradual incorporation of the viral envelope and viral M-protein into plasma membrane, as visualized in the protoplasmic and exoplasmic fracture (P and E, respectively) faces of the membrane; and (c) virus-dependent transient clustering of intramembrane particles at 4 degrees C, which is reversible after transferring the cells back to 37 degrees C. The following virus-induced phenomena occur only in the presence of CaCl2: (a) rounding of cells followed by their fusion; (b) transient decrease in the density of intramembrane particles; and (c) the virus induces uptake of 45CaCl2 by chicken erythrocytes. The uptake is specific as it is inhibited by LaCl3, and no accumulation of [14C]glucose-1-phosphate ([14C]G-1-P) could be observed under the 45 CaCl2 uptake conditions. The data show that fusion of virus with plasma membrane is a Ca++-independent process and, as such, it should be distinguished from the virus-induced membrane-membrane and cell fusion processes. The latter is absolutely dependent on the rise of intracellular Ca++, as reflected by the fact that Ca++-induced rounding of chicken erythrocytes always precedes fusion (Volsky, D. and A. Loyter. 1977.Biochim. Biophys. Acta 471:253--259).

Involvement of fusion activity of ultraviolet light-inactivated Sendai virus in formation of target antigens recognized by cytotoxic T cells

Mice inoculated with ultraviolet light-inactivated Sendai virus mount a cell- mediated immune response to the virus. Cytotoxic T cells specific for Sendai virus can be obtained by in vitro secondary stimulation of primed spleen cells with syngeneic stimulator cells coated with UV-inactivated Sendai virus. Neither in vivo nor in vitro stimulation alone is sufficient to generate specific cytotoxic T cells. Sharing of the H-2 haplotype between cytotoxic T cells and target cells is required for the Sendai virus-specific lysis to occur. The fusion (F) glycoprotein of Sendai virus has been implicated in target antigen formation (20). Ethanol treatment of Sendai virus causes complete inactivation of the cell-fusion and hemolytic activities of the envelope, but does not affect the antigenicity of the F glycoprotein; furthermore, hemagglutinin and neuraminidase activities of the envelope HANA glycoprotein are also left intact after ethanol treatment. Target cells can be prepared by coating them with various numbers of UV-inactivated Sendai virus that have been treated with ethanol or, as a control, phosphate-buffered saline (PBS). The amount of virus adsorbed to target cells during the cytotoxicity reaction time using either ethanol-treated or untreated (PBS "treated") virions is essentially identical, but target cells coated with ethanol-treated Sendai virus fail to serve as targets for cytotoxic T cells. These results indicate that fusion activity of the Sendai virus envelope is essential to the formation of the target antigen and that virus adsorption to cell surfaces without fusion of the envelope with cell membranes is not sufficient to allow killing by virus-specific cytotoxic T cells.

Lipid vesicle-cell interactions. I. Hemagglutination and hemolysis

The interaction of lipid vesicles (liposomes) of several different compositions with erythrocytes has been investigated. Lecithin liposomes, rendered positively charged with stearylamine, exhibit potent hemagglutination activity in media containing low concentrations of electrolytes. The hemagglutination titer is found to be a linear function of the zeta potential of the lipid vesicles. Hemagglutination is reduced when the surface potential of the cells is made more positive by pH adjustment or enzyme treatment. Similarly, hemagglutination is reduced by increasing concentrations of electrolytes. Hemagglutination is examined theoretically and is shown to be consistent with vesicle-cell interactions that are due to only electrostatic forces. Vesicles containing lysolecithin in addition to lecithin and stearylamine cause lysis of erythrocytes, provided the lipids of the vesicles are above the crystal-liquid crystal phase transition temperature. In addition, hemolysis requires close juxtaposition of the vesicle to the cell membrane; vesicles precoated with antibodies exhibit severely diminished hemolytic activities, only a small fraction of which can be attributed to a reduction in hemagglutination titer. Evidence is presented indicating that a single vesicle is sufficient to lyse one cell. With regard to hemagglutination and hemolysis, lipid vesicles of simple composition mimic paramyxoviruses such as Sendai virus.

Lipid vesicle-cell interactions. III. Introduction of a new antigenic determinant into erythrocyte membranes

The introduction of a new antigenic determinant, 2,4-dinitrophenyl-aminocaproyl-phosphatidylethanolamine (DNP-Cap-PE), into the surface membranes of intact human erythrocytes is described. Fresh cells were incubated in the presence of liposomes composed of 10% DNP-Cap-PE, 5% stearylamine, 20% lysolecithin, and 65% lecithin. Such liposome-treated erythrocytes are shown to be susceptible to immune lysis by anti-DNP serum in the presence of complement. Uptake of DNP-Cap-PE by erythrocyte membranes is also demonstrated by immunofluorescence using indirect staining with rabbit anti-DNP serum followed by fluroescein-conjugated goat anti-rabbit IgG and by electron microscopy using ferritin-conjugated antibody. Antigen uptake did not occur at low temperatures or from vesicles lacking lysolecithin and stearylamine. Fluorescence microscopy shows that the antigen-antibody complexes are free to diffuse over the cell surface, eventually coalescing into a single area on the cell membrane. Electron microscopy suggests that a substantial proportion of the lipid antigen is incorporated by fusion of vesicles with the cell membrane. There are indications that vesicle treatment causes a small proportion of cells to invaginate.

Analysis of a viral agent isolated from multiple sclerosis brain tissue: characterization as a parainfluenzavirus type 1

A virus originally isolated from cell cultures obtained by lysolecithin-induced fusion of human multiple sclerosis brain cells with CV-1 cells has been analyzed for its antigenic, RNA, and polypeptide compositions, and for selective biological properties. Our findings establish that this isolate, designated 6/94 virus, contains a 50S RNA genome and is, as yet, indistinguishable from Sendai virus in its antigenic and total polypeptide compositions. Despite these similarities, the 6/94 and Sendai viruses differ in certain phenotypic properties. 6/94 virus is markedly less cytocidal for chick fibroblasts, especially at 37 C and, after beta-propiolactone inactivation, it possesses a greater capacity for cell fusion and a lower toxicity than does comparably treated Sendai virus. In addition, 6/94 virus shows greater hemolytic activity.

Fusion of erythrocytes by Sendai virus studied by immuno-freeze-etching

Extensive fusion of human erythrocytes agglutinated by Sendai virus was observed after 30 s of incubation at 37 C. Electron microscopy of thin sections failed to reveal the presence of virions, viral fragments, or discrete viral antigens reactive with ferritin-labeled antibody at the sites of fusion. Immuno-freezeetching of membrane surfaces demonstrated the dispersal of viral envelope antigens from what appeared to be original sites of viral attachment. Virus-induced clustering of membrane glycoproteins was interpreted as resulting from interaction of viral antigens with membrane receptor proteins and forming the structural basis for fusion of membranes with one another.

Early events in cell-animal virus interactions

Images

Hemolytic interaction of Newcastle disease virus and chicken erythrocytes. II. Determining factors

Standard procedures have been described for measuring paramyxovirus-induced hemolysis. The choice of these procedures is based on the analysis of the behavior of eight different strains of Newcastle disease virus. Significant strain-specific differences in hemolytic activity have been found. The presence of at least two kinds of inhibitors of hemolysis in virus preparations necessitates the use of purified virus when comparisons of hemolytic activities are to be made. In addition, it has been stressed that both hemagglutination titers and hemolysis determinations provide only relative values. Thus, quantitative comparisons can be made only with results obtained on the same day and with the same erythrocyte preparation.