When isolated by means of an anti-polyoma tumor (T) antiserum, the major product from mouse cells productively infected by wild-type polyoma virus is a polypeptide of 100,000 apparent molecular weight as judged by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. In cells infected by NG-18, an hr-t mutant carrying a deletion of about 150 base pairs in the early region of the viral DNA, a T antigen species appears that comigrates with that of the wild-type virus. Comparisons of peptides after partial proteolysis reveal no differences between mutant and wild-type products. Both wild-type and mutant 100,000 products can be labeled in vivo with [(32)P]orthophosphate. An independent and more reliable estimate of the molecular weight of this protein using guanidine/Sepharose chromatography yields a value of 81,000 for both mutant and wild-type species. The apparent identity of wild-type and mutant products indicates that the deletion in NG-18 lies outside of the region encoding this major T antigen species. Immunoprecipitates from wild-type infected cells shows four bands in addition to the "100,000" band; these have apparent molecular weights of 63,000, 56,000, 36,000, and 22,000 by sodium dodecyl sulfate/polyacrylamide gel electrophoresis; the 56,000 and 36,000 species are phosphorylated. All four of these lower molecular weight bands are absent or drastically reduced in the immunoprecipitates from NG-18-infected cells.