AbstractAntiserum against the synthetic peptide Lys‐Arg‐Ser‐Arg‐His‐Phe, corresponding to the carboxy terminus of polyoma virus medium tumor antigen (medium T antigen), immunoprecipitates a protein of 36,000 daltons from polyoma virus‐infected and uninfected cell extracts treated with the sulfhydryl group reagent N‐ethyl‐maleimide. This protein appears to share an antigenic determinant with medium T antigen that is normally buried inside the protein or covered up by another protein or cellular structure. The two‐dimensional tryptic fingerprints of the 36K protein and of medium T antigen are apparently unrelated to each other. Antiserum against the octapeptide Ac‐Met‐Asp‐Lys‐Val‐Leu‐Asn‐Arg‐Tyr, including the amino‐terminal heptapeptide sequence of the simian virus 40 (SV40) large tumor (T) and small T antigens, cross‐reacts with polyoma virus large T antigen, which has an identical amino‐terminal heptapeptide sequence except that Lys is replaced by Arg and Asn by Ser. The problem of cross‐reac