Neutrophils are important effector cells of acute inflammation because of their potential capacity to synthesize various proinflammatory mediators, and inhibition of their production is expected to result in anti-inflammatory effects. In this study, we investigate the effects of the anti-inflammatory cytokines, interleukin-10 (IL-10) and IL-4, on prostanoid synthesis in human neutrophils. Neutrophils isolated from healthy donors constitutively produced a small amount of prostaglandin E₂ (PGE₂ ) without any stimulations, whereas they produced a large amount of PGE₂ after lipopolysaccharide (LPS) stimulation. IL-10 and IL-4 selectively inhibited their LPS-induced PGE₂ production. Inhibition by both cytokines occurred at an early stage of LPS stimulation. Anti–IL-10 treatment of LPS-stimulated neutrophils resulted in enhanced PGE₂ production. LPS-induced PGE₂ and thromboxane B₂ (TXB₂ ) production in aspirin-treated neutrophils was significantly inhibited by IL-10, IL-4, and NS-398. Moreover, IL-10 and IL-4 inhibited LPS-induced cyclooxygenase (COX) activity in neutrophils. Western blot and immunocytochemical analysis showed that COX-2 protein was clearly induced in LPS-stimulated neutrophils and that its induction was inhibited by both IL-10 and IL-4. Moreover, both of these cytokines inhibited COX-2 mRNA expression in LPS-stimulated neutrophils. These results raise the possibility that these two cytokines may both offer potent clinical utility as anti-inflammatory agents in the future.