Feline leukemia virus antigen is demonstrable by immunodiffusion with rabbit precipitating antiserum to purified feline leukemia virus. The feline leukemia virus antigen was found in the tissues of 25 of 33 cats with lymphosarcoma and of 5 of 13 cats with infectious peritonitis. Its presence was correlated with the occurrence of feline leukemia virus demonstrable by electron microscopy. The one clinically normal cat giving a positive test for feline leukemia virus antigen belonged to a household in which two cats had developed lymphosarcoma. With the exception of a dog with lymphosarcoma induced by feline leukemia virus, the antigen was absent from lymphosarcoma and nonlymphomatous tumors of other species (man, dog, cow, goat, or pig).