In addition to the virus-encoded large T and small T antigens, several polypeptides with molecular weights between 48,000 and 55,000 are found in immunoprecipitates obtained from extracts of SV40-infected cells or SV40-transformed cells, using the sera of tumor-bearing animals (Gaudry et al. 1978; Melero et al. 1979; Lane and Crawford 1979). Reconstruction experiments and tryptic fingerprinting of the 48K–55K proteins ruled out the possibility of their being proteolytic degradation products of large T antigen (D. T. Stitt et al., in prep.). In SV40-infected monkey cells, the appearance of a 55K protein was not affected by deletions between 0.59 and 0.54 map units of the viral genome, indicating that small T antigen is not involved in its induction. In the same system, the stability of the 55K protein was not affected in tsA-infected cells at the nonpermissive temperature, suggesting that the A gene does not encode the 55K protein (Melero et...