Cytotoxic T (thymus)-lymphocytes (CTL) with specific cytotoxicity against the leukemia-associated antigens of FBL-3, a syngeneic Friend virus-induced leukemia in C57BL/6 mice, could be adoptively transferred to sublethally X-irradiated (350 R) syngeneic hosts and could be induced by adoptive transfer of either normal or presensitized lymphocytes obtained from immunocompetent hosts. The CTL and their precursor cells were systemically distributed in peripheral lymph nodes and spleen, although they had the tendency of homing to the lymphoid tissue of the same origin. Direct cytotoxicity was obtained with the lymphocytes from these lymphoid tissues, and cells obtained from these lymphoid tissues could produce secondary cytotoxic responses by the mixed lymphocyte tumor cell culture reactions 40--60 days after adoptive transfer. In addition, lymph node and spleen cells had a synergistic effect on the induction of cytotoxicity. These findings indicated that tumor immunity was widely distributed and that various populations of lymphocytes were involved in the generation of efficient cell-mediated cytotoxic responses.