The popliteal lymph node cells of immunocompetent mice generated a strong in vitro cytotoxic response to footpad injection of several human tumor cell lines and the resulting mouse effector cells predominantly used a perforin-mediated cytotoxic mechanism. A relatively minor FasL-dependent cytotoxic response to CEM-CCRF and Jurkat leukemias, but not colon carcinoma COLO 205 cells, was also detected in immunized perforin-deficient mice. In vitro depletion of CD3+ CD8+ T cells, but not CD4+ T or NK1.1+ cells, completely inhibited lysis of human tumor cells, suggesting that CD3+ CD8+ T cells were effectors of perforin-mediated xenospecific cytotoxicity. Xenospecific cytotoxic T cells from wild-type mice were extremely efficient at rejecting tumor when adoptively transferred into scid mice bearing established COLO 205, CEM-CCRF, or Jurkat tumor xenografts. By contrast, cytotoxic T lymphocytes of perforin-deficient mice had no effect on the growth of established tumor xenografts. These data indicate that perforin, and hence direct cytotoxicity, plays a key role in the ability of adoptively transferred CD8+ cytotoxic T lymphocytes to eradicate established xenografts.