Antisera to several different cytoplasmic frog virus isolates were prepared in rabbits. Comparison of eight virus isolates by means of 50% plaque reduction tests revealed no antigenic differences. In neutralization kinetic studies of three isolates, LT-1, LT-2, and FV-1, very low neutralization rate constants (K = 0.18-2.9) and high persistent fractions of nonneutralizable virus (14-38%) were obtained. No antigenic differences were detected, although one isolate, LT-2, consistently gave lower K values and higher persistent fractions with homologous and heterologous sera than did LT-1 and FV-1. It is concluded that all the cytoplasmic frog viruses studied, originating from normal and tumor-bearing frogs from widely separated parts of the United States, comprise a single serotype.
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