Immunological differences between tumors induced by either chemical carcinogens or viruses and the tissue from which such tumors originate have been demonstrated in almost all systems in which this feature was investigated.1,2 In addition, antigenic properties have been shown for spontaneous experimental tumors of various histological types.3 Antigens designated as a specific character of the neoplastic cells have been demonstrated by presensitization of an isologous host which then is relatively resistant to subsequent challenges with the transplanted tumor. This sensitization usually is achieved by excision of tumor after a short period of growth, irradiation of an established tumor, or injection of lethally irradiated tumor cells. The immunological basis for this resistance in chemically induced tumors has been found by demonstrating the in vitro activity of immune lymphocytes on tumor cells4,5 or by adoptive transfer of immunity to syngeneic animals. Serological tests for demonstrating antibody against these tumors