A transplantable, murine reticulum cell sarcoma is described which exhibits the cytologic, adherence, and phagocytic properties of macrophages. It forms specific rosettes with erythrocytes in the presence of the corresponding anti-serum. The ascites cells mediate antibody-dependent cellular immunity as assayed by release of radioactivity from 51Cr-labeled erythrocytes. The contribution of contaminating host cells in the cytotoxic reaction was ruled out by growing the tumor in F1 mice and removing the host cells by anti-H2 serum and complement. The tumor cells have receptors for IgG2a and IgG2b immunoglobulins. The availability of a pure population of effector cells in the immune system allows study of the biochemical processes pursuant to lysis of foreign cells.