Adoptive transfer of lymphocytes was performed into mice bearing progressor Moloney murine sarcoma virus (M-MSV)-induced primary tumors. Lymphocytes from mice immunized against M-MSV (designated as immune lymphocytes, IL) inhibited tumor growth when transferred before the virus injection. However, the same lymphocytes were ineffective against tumors consisting of a solid mass when transferred after the virus inoculation. When IL was transferred shortly after 500 rad whole-body irradiation of the tumor-bearing mice, tumors regressed markedly and most of them became undetectable, while tumors of other groups receiving normal lymphocytes or medium combined with irradiation ceased to regress soon after the treatment. This effect of IL was abolished by the treatment of IL with anti-Thy 1 antibody and complement, and was not observed against antigenically distinct syngeneic tumors (Meth-A). Concomitant transfer of spleen cells from mice bearing progressing tumors with IL partially impeded the effect of IL. These results suggest that specific immune T lymphocytes induce tumor regression when transferred after the whole body irradiation of the tumor-bearing mice. The effects of irradiation may be divided into two categories: the direct effect on tumor cells, and the indirect effect upon the activity of IL.