Simian-virus-40-specific T-antigen was isolated by immunoprecipitation. From other studies we have proof that the T-antigen described in this work is coded by the viral DNA. The molecular weight estimated from electrophoretic mobility in sodium dodecyl sulfate-polyacrylamide gels of T-antigen isolated from nonpermissive mouse cells in abortive infection is 86,000 and from permissive monkey cells in lytic infection is 82,000. The 86 kilodalton T-antigen is readily converted in vitro into an 82 kilodalton form by incubation with extracts from permissive monkey cells but not with extracts from nonpermissive mouse or hamster cells. This and the results of fingerprinting analysis of tryptic peptides suggest that T-antigen may be processed in permissive cells.