The roles of tumor necrosis factor (TNFα) and reactive nitrogen intermediates (RNI) as effectors of macrophagemediated tumor cell killing were investigated in a variety of tumor cell lines. Three TNFα‐sensitive tumor targets were also susceptible to resting bone‐marrow‐derived mononuclear phagocytes (BMMP). This macrophage lytic activity was markedly diminished or even abolished by anti‐TNFα, indicating that TNFα is the major effector of macrophagemediated killing of these targets. The other 21 tumor cell lines examined were resistant to TNFα but, in their large majority, were more or less susceptible to killing by interferon γ (IFNγ)‐ and Corynebacterium parvum (CP)‐activated BMMP. Among the various analogues of L‐arginine used to assess the role of L‐arginine‐derived RNI as mediators of macrophage tumoricidal activity, N^G‐monomethyl‐L‐arginine (NMMA) was most efficient in suppressing RNI secretion by activated macrophages. In some macrophage tumor‐cell combinations, NMMA inhibited both the generation of RNI and the expression of tumoricidal activity in a dose‐dependent manner, suggesting a central role for RNI as effectors. In other combinations, NMMA in concentrations that abolished secretion of RNI either affected tumor‐cell killing only after its induction by IFNγ, or not at all. The findings not only support the thesis that macrophages possess various means of coping with tumor cells but also suggest that the mechanism becoming operative is determined predominantly by the pathway of macrophage activation and the properties of the tumor‐cell type.