The murine leukemia L12lO-specific cytotoxic T-cell clone K7 was established by repeated antigenic stimulation of spleen cells of LIZIO-immune CDZF, mice in long-term culture. K7 possessed L 1210-specific cytolytic activity detectable by the slCr-release test, but lost its cytolytic activity about 100 days after initiation of culture (designated as clone K7L). Clone K7L retained i t s L 1210-specific tumor-growth-inhibitory activity independently of culture supplementation with IL-2. K7L possessed the cell-surface antigenic phenotype of cytotoxic T cells, Thy-l+, Lyt-I-. Lyt-2+. This clone K7L displayed surprisingly strong activity in specifically inhibiting in vivo tumor growth of LIZ10 by day 5 in the peritoneal cavity of CD2F I mice when injected together with 10s tumor cells (more than 9Ooh inhibition at E/T = 5). and prolonged survival times of most of the mice for more than 60 days, whereas control mice inoculated with LIZ10 alone died on day 9-17. This unique in vivo anti-tumor activity was not correlated to the in vitro cytolytic activity. Nevertheless, bystander inhibitory effect on the growth of thirdparty tumor cells (P388) was not seen in mice injected with a mixture of LIZ10 and P388 together with K7L, suggesting that K7L inhibited LIZ10 growth by direct cell-to-cell interaction. Host cells such as X-irradiation (800R)-sensitive lymphocytes and Carrageenan-sensitive macrophages were not involved in the early growth inhibition of LIZ10 by K7L.