The ability of cells from primary MSV-induced tumors to function as effector cells in vitro was evaluated. Host cells were isolated by enzymatic disaggregation of the tumor and fractionated by sedimentation velocity at unit gravity on a Ficoll gradient. Characterization of these cells indicated that 30 to 40 % were T lymphocytes, about 50% were macrophages and less than 5% were B lymphocytes. Two different functional activities were mediated by these cells: cytolysis, as measured by the CRA, and inhibition of proliferation, as measured by the GIA. The effector cells in the CRA were T cells with sedimentation velocities of 3.5 to 4.0 mm/hr, whereas those cells which mediated the GIA were presumably macrophages and displayed a heterogeneity in size two peak sedimentation velocities, one at 4.0 mm/hr and another at 6.0 mm/hr. Activity by the effector cells in the CRA was antigen specific in contrast to the activity in the GIA which was directed against cells which did not carry detectable cross-reacting antigens.