Resident mouse peritoneal macrophages (M phi) produced significant amounts of superoxide anion (O2-) in response to phagocytic stimuli. When M phi were exposed in vitro for 20 hr to fibroblast interferon (IFN-beta), their capacity to release O2- was significantly reduced, such reduction being more evident with increasing IFN-beta concentrations. In contrast, O2- production by M phi exposed for 20 hr to the lymphokine macrophage activating factor (MAF) or treated with either MAF or IFN-beta for 4 hr was not significantly different from that of control cells. This pattern of activity closely followed that of M phi-mediated suppression of lymphocyte proliferation, which was dramatically reduced by 20 hr exposure of M phi to IFN-beta, but unchanged by treatment with MAF. No correlation was however found between superoxide anion generation and enhancement of tumoricidal capacity in IFN-beta-treated M phi. We thus concluded that O2- does not play a relevant role in IFN-beta-induced M phi cytolysis, whereas the reduction of O2- production could be of major importance in the decrease of M phi suppression induced by IFN-beta.