Five murine macrophage or monocyte-related tumor cell lines in culture were compared for antibody-dependent lysis (ADL) of a tumor target (T lymphoma EL4) and lysis and phagocytosis of sheep erythrocytes (RBC). Some lines were effective only with RBC targets, while others were capable of lysing these and tumor targets. Both murine alloantisera to H-2 and Thy 1.2 antigens on the target cells and rabbit anti-mouse spleen and anti-mouse thymus sera directed target lysis, while normal mouse or rabbit sera were inactive. Preincubation of macrophage line RAW264 with lipopolysaccharide (LPS) or purified protein derivative from Mycobacteria (PPD) for 2 days resulted in an average of 76% or 86% increase, respectively, in ADL of RBC, although there was no stimulation in RBC phagocytosis or in nonspecific or antibody-dependent lysis of tumor targets. These results indicate that macrophage cell types are capable of antibody-dependent tumor lysis, that macrophages are probably heterogeneous in effector cell activities, and that they can be stimulated to increased ADL in the complete absence of other cell types.