The ability of alloimmune spleen cells expanded in mixed leukocyte culture (MLC) and cloned cytotoxic T lymphocytes (CTL) to kill H-2-compatible leukemia in vivo was evaluated. In comparison with fresh alloimmune spleen cells, MLC-expanded cells had a significantly higher frequency of CTL reactive against leukemia targets in vitro. However, the reactivity of MLC-expanded cells against first-passage spontaneous AKR (H-2k) leukemia in vivo was significantly less than when an equivalent number of fresh alloimmune spleen cells was injected. Comparable antileukemia reactivity was observed in vivo only when the inoculum of MLC-expanded cells was 2-3-fold higher than that of fresh spleen cells. This relative ineffectiveness was attributed to the altered migration pattern of cultured cells in vivo. IL-2-dependent cloned CTL, specific for a normal lymphocyte antigen (Qa-1b) also present on leukemia cells, were derived from MLC-expanded cultures and tested in vivo. For cloned CTL, as with MLC-expanded cells, eradication of AKR leukemia in vivo was associated with the tissue distribution pattern of the injected effector cells. That is, an effective antileukemia reaction was achieved only in tissues in which effector and target proximity was maintained. Qa-1b-specific cloned CTL did not interfere with engraftment of autologous or allogeneic bone marrow in lethally irradiated host mice, nor did they cause any clinically evident graft-versus-host disease. These findings suggest that cloned CTL specific for a normal cell surface antigen with limited host tissue distribution, but present on tumor cells, could be used for adoptive immunotherapy, provided CTL and tumor cell proximity can be attained.