Takao Kodama, and Hiroshi Kobayashi 2

duced transplantable fibrosarcoma KMT-17 cells was investi-gated in relation to days after ip transplantation. Cytotoxicity tests with antiserum against tumor-associated surface antigen of KMT-17 cells revealed that cytotoxic sensitivity and absorb-ing capacity decreased after transplantation, but they in-creased when other normal rats were given transplants of tumor cells. A decrease in the sensitivity was observed when immunosuppressively irradiated rats were given tumor trans-plants. Tumor cell density in the abdominal cavities of rats di-rectly influenced the cytotoxic sensitiVity _ However, the sensi-tivity and absorbing capacity of KMT-17 cells to antiserum against the histocompatibility antigen did not change after transplantation. The possible mechanisms of antigenic change of KMT-17 cells were discussed.-J Natl Cancer Inst 55: 1079-1083, 1975. Despite the presence of tumor-specific antigens, tumor cells grow progressively in immunologically competent hosts. Tumor cells may escape from the defense mecha-nism of the host because the tumor-specific antigen is not recognized if it is weak or unstable in expression. Antigenic expression of tumor cells is not always stable in vitro (1-6). However, it is not known whether the an-tigenic expression of tumor cells is always stable in vivo. We have obtained antiserum against the tumor-associ-ated surface antigen (TASA) of KMT-17 cells by immunizing syngeneic rats with Friend virus-infected KMT-17 (FV-KMT-17 or xenogenized KMT-17) cells, which are never lethal in syngeneic rats (7-14). Immuni-zation with FV-KMT-17 cells may have helped produce high-titered antibody against TASA of KMT-17 cells. Therefore, we designed these experiments to investigate the expression of T ASA and histocompatibili ty antigen of KMT-17 cells after transplantation in vivo. MATERIALS AND METHODS Rats.-Inbred WKAjMk rats were offspring of parents maintained by consecutive brother X sister mating for more than 200 generations in the Experimental Anima