AbstractA strong cell‐mediated immune response against Friend, Moloney, Rauscher virus‐induced (FMR) cell surface antigens has been demonstrated previously in mice which reject oncornavirus‐induced tumors. In order to identify an eventual suppressor mechanism in animals with progressively growing tumors, experiments were initiated in C 57 BL/6 mice bearing either a murine sarcoma virus (MSV) tumor or Moloney virus‐induced lymphoma (MBL2). Progressive tumor growth was induced (a) in viremic animals first infected with Moloney murine leukemia virus (M.MuLV), then inoculated as adult with MSV; (b) in nonviremic animals injected with MBL2 lymphoma cells. In the absence of tumor cells, viremia induces specific tolerance for which there is no evidence for suppressor cells. In tumor‐bearing mice, specific suppressor T cells are detected which are able to inhibit the generation of anti‐FMR cytolytic T lymphocytesin vitroand enhance the tumor growthin vivo.In addition to the specific suppressor T cells, a nonspecific suppressive activity mediated by metastatic T lymphoma cells is demonstrated in the spleens of lymphoma‐bearing animals. The respective role of the virus and tumor cells in the induction of tolerance to M.MuLV‐induced antigens, and their relationship to other components of the specific cell‐mediated immune resp