One or 4 weeks after grafting of a syngeneic sarcoma (T2) to C57BL/6 mice, lymph-node cells (LNC) are cytotoxic in vitro for the cells of this tumour. But after 2 weeks LNC are not cytotoxic or show a non-significant activity. These second-week LNC, added to cytotoxic lymphocytes (CL) from the first or fourth week, reduce considerably the cytotoxicity of the latter cells. When velocity sedimentation at unit gravity is used to fractionate 11th-14th day LNC, some fractions, enriched in small lympoid cells, kill the cancer cells. Other fractions, containing large blast cells, lack this property but can suppress the activity of the small cells or of fourth week CL. These suppressor cells can also be separated from active CL by passage on a glass bead column. They are "adherent" while CL are non-adherent at this stage. This suppressor effect is abolished when the lymphoid cell suspension is treated by anti-theta serum, but macrophage depletion does not modify the inhibition. The cell responsible for the suppressor effect is thus a large T lymphoid cell, adherent and non-phagocytic. It seems to act essentially on an effector phase of the cell-mediated cytotoxicity.