F1 lymphocytes stimulated in vitro by parental cells were evaluated for cytotoxicity against semisyngeneic tumors and lymphoblasts. B6AF1 (H-2a,b) spleen cells were placed in culture with C57BL/6 (H-2b) or B10.A (H-2a) cells and 6 days later were assayed for cell-mediated cytotoxicity in vitro; also subcutaneous, intraperitoneal, and intrapulmonary tumor neutralization experiments were performed. F1 lymphocytes sensitized by parental cells showed high levels of cytotoxicity to the tumor cells of the parental haplotype but no lysis of parental blastoid cells. Tumor cells from irrelevant haplotypes were also lysed. The cells mediating in this type of killing were characterized phenotypically as Thy-1.2- and Lyt-2.2-positive. In a subcutaneous tumor neutralization test (Winn assay), marked suppression of EL-4 lymphoma (H-2b) and B16 melanoma (H-2b) was observed. Likewise, tumor control was seen in an intraperitoneal tumor model. These studies show that F1 versus parental sensitization can be used to lyse tumor in vitro and in vivo and should be explored as an immunotherapeutic tool.