Antitumor activity of spleen cells from BALB/c mice bearing RL male 1 lymphoma was studied. In the Winn assay spleen cells of the tumor-bearing mice inhibited the growth of RL male 1 lymphoma. This antitumor activity of spleen cells was not detected by the in vitro cell-mediated cytotoxicity assay (4-h 51Cr release assay). The effector cells in spleen cells were T-cells which manifested asialo GM1 on their cell surfaces and were radiosensitive (1000 rads). The analysis of T-cell subsets using Lyt markers showed that the effector cells expressed the Lyt-2 antigen with a small amount of the Lyt-1 antigen on their cell surfaces. In addition antitumor activity of spleen cells of the tumor-bearing mice was weak in the early stage of tumor growth, strong in the mid-stage, and disappeared in the late stage. The mechanisms of antitumor activity of spleen cells of the tumor-bearing mice are discussed.