Objective: To study the effect of interleukin (IL)-4 and IL-10 on nitric oxide (NO) production by macrophages.¶Materials and Methods: Elicited or resident peritoneal macrophages (PMO) and a macrophage cell line Raw264.7 were primed by IL-4 or IL-10 for 6 hours, and were further incubated in the presence of interferon (IFN)-γ and/or lipopolysaccharide (LPS) for 48 hours. NO₂ ^- accumulation in the supernatant of cultured cells was used as an indicator of NO production and was determined by the standard Griess reaction adapted for microplates. The amount of tumor necrosis factor (TNF)-α in the culture supernatants was determined with a commercially available ELISA kit. The absorbance was measured at 450 nm with a microplate photometer.¶Results: IL-4 inhibited NO production by murine macrophages of different sources and the macrophage cell line Raw264.7. In contrast, different macrophage populations showed differential responses to IL-10. After stimulation with LPS or IFN-γ, IL-10 suppressed NO production by elicited PMO but enhanced NO production by resident PMO or by Raw264.7. Both IL-4 and IL-10 inhibited the production of TNF-α, which has been shown to play a crucial role in NO production. In the presence or the absence of blocking antibody to TNF-α, IL-10 always enhanced NO production by resident PMO. This result suggests that the inhibition of TNF-α production and the enhancement of NO production by resident PMO stimulated with IL-10 are independent, coexisting events.¶Conclusions: Factors other than TNF-α have been suspected to influence NO production by macrophages, and this study indicates that IL-10 may be a candidate cytokine for resident PMO.