Human mononuclear phagocytes (monocytes and macrophages) act as effectors in the destruction of tumor cells. Peritoneal macrophages from normal or infertile women killed a variety of tumor cells in vitro more efficiently than did blood monocytes from the same subjects. Lysis depended on the effector-to-target cell ratio and was neither reproduced by supernatants from nor lysates of the mononuclear phagocytes. Normal fibroblasts were not lysed. Lipopolysaccharide (10(1)-10(4) ng/ml) did not alter the monocyte- or macrophage-mediated tumor cell killing. The monocytes and macrophages had equivalent basal and phorbol 12,13-myristate acetate-stimulated H2O2 and O-2 production, and the reactive oxygen species scavengers or quenchers catalase, superoxide dismutase, mannitol, and L-histidine did not diminish the killing. These observations suggest that the spontaneous tumor cell killing by human mononuclear phagocytes was not mediated by reactive oxygen species.