Interleukin‐10 (IL‐10) has been reported to inhibit nitric oxide (NO) synthesis and microbicidal activity of interferon‐γ (IFN‐γ)‐stimulated macrophages (MΦ) by preventing the secretion of tumor necrosis factor‐α (TNF‐α) which serves as an autocrine activating signal. We have examined the effects of recombinant IL‐10 on the capacity of IFN‐γ together with exogenous TNF‐α to induce NO synthesis by bone marrow‐derived MΦ. Under these conditions and in contrast to its reported deactivating potential, IL‐10 strongly enhanced NO synthesis measured as nitrite (NO) release (half maximal stimulation at approximately 10 U/ml). IL‐10 further increased NO production by MΦ stimulated in the presence of optimal concentrations of prostaglandin E₂, a positive modulator of MΦ activation by IFN‐γ/TNF‐α. Increased steady state levels of NO synthase mRNA were observed in 4‐h IFN‐γ/TNF‐α cultures and enhanced NO release was evident 24 h but not 48 h after stimulation. These results suggest that the effects of IL‐10 on MΦ function are more complex than previously recognized.