Mixtures of lymph node and spleen cells from normal (untreated) BALB/c mice and from BALB/c mice whose syngeneic tumors had been excised 7-28 days previously ("tumor-excised mice"), were sensitized in vitro by cultivation for 9 days with cells from syngeneic, methylcholanthrene-induced sarcomas. The in vitro-sensitized lymphoid cells were tested in a 36-h microcytotoxicity assay for reactivity against target cells carrying the sensitizing tumor antigens, as well as against control target cells lacking these antigens. After co-cultivation with tumor cells, lymphoid cells from both normal and tumor-excised mice were cytotoxic to tumor cells carrying the sensitizing antigens. The cytotoxicity was generally specific, and occurred at low effector: target cell ratios (in some experiments down to 1:1). When lymphoid cells from tumor-excised mice were exposed in vitro for 9 days to cells carrying the same antigens as those which were originally present on the surgical excised tumors, the effector cells obtained gave a dose-dependent cytotoxic response suggestive of a linear relationship. When lymphoid cells from normal mice were similarly sensitized for 9 days, specifically cytotoxic lymphoid cells were generated but no linear dose-dependent response was detected.