Using antisera obtained from rats bearing Rous sarcoma virus-induced tumors, we have identified in chick embryo fibroblasts transformed by a non-defective Rous sarcoma virus a protein of molecular weight 56,000, not found in cells infected by transformation defective mutants with a partial or complete deletion of the src gene, or in cells infected with a non-transforming avian leukosis virus. This protein appeared to be specific to cells transformed by Rous sarcoma virus. In studies on SV40 early antigens, Ad2 ion ND2 induced in infected cells two SV40 specific polypeptides of 42,000 and 56,000 molecular weight; Ad2 ion ND1 induced a 28,000 molecular weight peptide. The Ad2 ion ND1 segment is sufficient to induce the SV40-specific tumor antigen in mice.