Polyoma-infected 3T6 cells contain a number of proteins precipitable by serum from rats carrying polyoma-induced tumors. The virus codes for three species having apparent molecular weights of 90,000, 60,000 and 22,000 daltons, as determined by polyacrylamide gel electrophoresis (90K, 60K and 22k). The 90K and 22K species produced by a large plaque and a small plaque wild-type polyoma have similar mobilities, but the 60K species produced by the large plaque wild-type. In cells infected by each of seven polyoma tsA mutants, the 90K species is unstable at the nonpermissive temperature, while the 60K and 22K species are stable. In cells infected by a mutant carrying a deletion between roughly 98 and 3 map units in the early region of the viral genome, the 22K species is present, but the 90K and 60K species are absent. Tryptic peptide analysis of the isolated 90K, 60K and 22K species shows that the three species have common N terminal regions. The 60K and 22K species contain amino acid sequences not found in the 90K species , and the 60K species has several unique, methionine-containing peptides not found in either the 22K or 90K species. Two polyoma-transformed BHK cell lines do not have detectable amounts of the 90K protein.