Radioactive labeling of proteins from guanidine-hydrochloride disrupted feline leukemia virus was done in vitro by attaching [(3)H]methyl groups to protein amino groups with reductive alkylation. When in vitro labeled disrupted virus was analyzed by gel filtration in the presence of 6 M guanidine-hydrochloride the expected six major and one minor protein peaks were detected. When the same preparation was analyzed by immunodiffusion, the three antigens tested were found to have retained their antigenic activity.