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.