AbstractPrevious reports have demonstrated that adult C 57 BL/6 mice infected with murine sarcoma virus (MSV) develop a strong cell‐mediated immune response against Friend, Moloney, Rauscher virus‐induced type‐specific (FMR) antigens and reject their tumors. To demonstrate a possible role for auto‐anti‐MSV T blasts, syngeneic C 57 BL/6 mice were immunized with highly enriched anti‐FMR cytolytic T cells. One of 3 pools of these autoimmune T cells prepared from 12 surviving immunized mice (a) inhibited specifically thein vitroanti‐MSV cytolysis generation and (b) enhanced drastically the MSV tumor growthin vivo.The possibility for such an immunization procedure to induce anti‐idiotype T cells, the repeatability of this effect and the relationship of the suppressor cells with antigen‐specific suppressor cells and other components of the anti‐MSV immune res