We previously reported that rat tumor cells infected with Friend virus were not transplantable as continuously growing neoplasms in otherwise susceptible strains of rats because they possessed Friend virus-related antigens. In the present investigation rat tumor cell lines WST-5, DLT, and KMT-68 did not grow in otherwise susceptible rats if they had received injections of the Friend virus-infected cells of these same lines before challenge. The immunizing effect of the virus-infected cells on the transplanted noninfected tumor cells was notably greater than the effect of other immunizing procedures we had tested. Even when the same immunizing methods were used, the immunogenicity of tumor cells infected with Friend virus on the transplanted noninfected tumor cells was definitely stronger than that of noninfected tumor cells. The possibility of immunologic treatment of tumors with virus-infected tumor cells is discussed.