Studies were made for biologic characterization of the ES4 strain leukosis virus with respect to quantitative host response and antigenic properties studied by the neutralization reaction in comparison with the properties of other avian tumor virus strains. In Line 15 White Leghorn chicks, the median log latent period was closely related to log dose which yielded procedures like those useful with strains BAI A and R for titrating the agent. Older chicks to 60 days, as with strain R but contrary to RPL 12 and BAI strains, were more responsive than 5-day-old birds. The ID50 was 104.9 ES4 virus particles per inoculum in comparison to analogous values of 105.7 and 107.5 for R and BAI A, and the chicken response to ES4 was more homogeneous than to R and far more so than to BAI A in the same line of chickens. In cross-neutralization tests, ES4 was reciprocally related to BAI A; ES4 immune serum was effective against R but little reverse activity occurred; and there was no reaction with RPL 12 strain. ES4 virus was neutralized by anti-chick tissue rabbit immune serum (like BAI A and R) but not by Forssman (like R but unlike BAI A) antibodies. Although ES4 strain was either directly or indirectly related to the other strains, it was not identical with any. A comparison of the antigenic behavior of avian tumor viruses studied revealed differences among all, but familial interrelationships were evident.

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