Spleen cells from mice inoculated with syngeneic murine sarcoma virus (MuSV)-transformed tumor cells suppressed the mixed leukocyte reaction. Mice inoculated with virus-producing tumor cells demonstrated two types of suppression. First, an early suppression was shown to be mediated by an adherent suppressor T-cell on the basis of its sensitivity to Thy 1.2 antiserum plus complement and the absence of the early suppression in T-cell-deficient nude mice. This suppression may have been induced in response to viral antigens associated with cell surface antigens (modified self-antigens) or viremia, because it was not induced by a non-virus-producing tumor cell line. Second, a late suppression was also seen in tumor-bearing BALB/c mice. This suppression was shown to be T-cell-independent by its presence in nude mice and by its resistance to gamma-irradiation and Thy 1.2 antiserum plus complement. In addition, the late suppression was present in mice inoculated with a nonproducer clone of MuSV-transformed cells. This finding suggests that viral antigens and/or viremia is not required for induction of the late suppression.