The interaction between target cells (mouse sarcoma cells) and syngeneic immunocytes (peritoneal cells) in close proximity but without direct contact was studied. The two cell types were separated by Millipore membranes, which allow diffusion of substances of large molecular size, in an assembly which permits cultivation of target cells on one side of the membrane and immunocytes on the other side. When brought into proximity in this manner for 48 to 72 hours, immunocytes from donors which had been immunized against syngeneic tumors caused destruction of the target cells. Since serum from immunized donors had no effect, it appears that the immunocytes produced a diffusible cytotoxic substance (or substances) which may be different from typical antibody.