Sensitized lymphocytes have been demonstrated to inhibit tumor growth in vivo systems. In SV40-viral oncogenesis, some specified immunization with virns and with virus-induced tumors to the animals results in rejection of further transplantation of the tumor cells. To analyze these phenomena by our specially devised cytotoxicity test, the authors have studied cell mediated immunity for tumor specific transplantation antigen (TSTA) in SV40 viral oncogenesis. The results suggest that sensitized lymphocytes inhibit the growth of heterogenic SV40-transformed target cells as well as syngenic ones. And the lymphocytes from tumor-bearing animals have lower ability of the inhibition on the target cells, which may be explained that the lymphocytes in tumor bearing animals are under the condition of immunologically insufficient state.