A series of murine macrophage cell lines was assayed for killing of B and T lymphocytic and myeloid tumor targets by radiolabel release at effector:target ratios of 20:1. One group of lines was inactive in all assays. Another group of lines showed moderate spontaneous cytotoxicity to lymphoid tumors that was greatly enhanced by inclusion of antibody, lipopolysaccharide, or phorbol myristate acetate in the 22-hr assays. Addition of lymphokine to the assays induced only moderate killing. Pretreatment of cell lines with lipopolysaccharide, phorbol myristate acetate, lymphokine, or nonlymphocyte sources of macrophage colony-stimulating factors did not activate "angry" or nonspecific killing. However, such pretreatment greatly stimulated antibody-dependent cellular cytotoxicity. Cytotoxicity was not due to crowding or poor culture conditions: macrophage line supernatants were not toxic; rapidly growing lymphoid lines used in place of macrophages did not kill; and high macrophage concentrations (10(6)/ml) had reduced cytotoxic activity. These experiments show that the same cell type can mediate both activated killing and antibody-dependent cellular cytotoxicity of tumor targets. The active macrophage lines appear to be qualitatively similar to each other and to peritoneal exudate populations in tumoricidal activity. We showed previously that these macrophage lines could also phagocytose and lyse antibody-coated red blood cells. The pretreatment experiments suggest that the antibody-dependent cellular cytotoxicity state can be regulated independently of macrophage nonspecific tumoricidal capacity. These culture line models of macrophages offer several advantages in the analysis of cytotoxicity.