A cytotoxic T-lymphocyte clone derived from mice with progressively growing tumors. Tumor-specific T-cell clones were derived from spleen cells of mice bearing a syngeneic PHS-5 tumor (a P815 mastocytoma mutant). Cells were expanded in vitro and characterized and assayed for activity against the relevant tumor in vivo. Clone cells were CD4-, CD8+ T lymphocytes, as determined by fluorescence activated cell sorting analysis and were specifically cytotoxic against P815 tumor cells in vitro, as shown in chromium 51 release assays. These cells require both antigen and interleukin 2 to proliferate; neither alone is sufficient, even with the addition of interleukin 1. In an experimental P815 liver metastasis model, the adoptive transfer of GD11 or GD11.17 clone cells and injection of recombinant interleukin 2 (7500 U intraperitoneally) 3 days after infusion of tumor cells reduced the number of tumor nodules, while the adoptive transfer of lymphokine-activated killer cells was ineffective. 