T-antigens from simian virus 40 (SV 40)-transformed and lytically infected cells have been isolated by immunoprecipitation and their molecular weights estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. T-antigen from SV40-transformed mouse and hamster cells has an apparent molecular weight of 94,000 whereas that from several lines of SV40-infected monkey cells is 84,000. In a wheat germ cell-free system, mRNA from either transformed or productively infected cells is translated into a 94,000 species. Experiments with the protease inhibitors L-l-(tosylamide-2-phenyl)ethylchloromethyl ketone HCl and N-alpha-p-tosyl-L-lysylchloromethyl ketone HCl suggest that the 84,000 species of T-antigen found in infected cells is derived from the larger species by proteolytic cleavage. Further, the cleavage pathway probably involves a two-step reaction with an 89,000 intermediate. The biological significance of the two molecular weight forms of T-antigen is unknown, but the possibility that they have different physiological activities is discussed.