The abilities of various bacteria to induce in a pure population of bone marrow-derived mononuclear phagocytes (BMM phi) tumoricidal activity and/or the generation of reactive nitrogen intermediates (RNI) were comparatively assessed. Interaction of BMM phi with bacteria led to expression of these functional activities, indicating that the organisms were recognized as foreign. As the majority of bacteria elicited in BMM phi either tumoricidal activity (that is maintained for days) or the production of RNI, measured by the release of nitrite (that is short-lived), it appears that the two functions are under separate control. However, both functions are inhibited or even abrogated by arginase or the L-arginine analogue, NG-monomethyl-L-arginine, suggesting that their expression is dependent on L-arginine.