In previous studies it has been shown that the bacterial endotoxin LPS induces an initial burst of inflammatory cytokine synthesis in human monocytes, which is followed by substantial IL-10 production; the IL-10 then down-regulates the inflammatory cytokine production as well as IL-10 production itself. Herein we tested the hypothesis that IL-10 production in human monocytes is under control of one of the cytokines induced by LPS. Accordingly, we cocultured purified human peripheral blood monocytes with a panel of cytokines including TNF-alpha, IL-1 alpha, IL-1 beta, IL-6, granulocyte macrophage-CSF, transforming growth factor-beta, and IFN-alpha and then measured IL-10 mRNA production using a semiquantitative reverse transcription-polymerase chain reaction technique. We found that TNF-alpha had a major effect on IL-10 mRNA production, inducing a 20- to 120-fold increase over baseline production. In contrast, IL-1 alpha, IL-1 beta, IL-6, granulocyte macrophage-CSF, transforming growth factor-beta, and IFN-alpha had little effect (< 3-fold). The induction of IL-10 mRNA by TNF-alpha in monocytes was dose dependent and began between 8 and 24 h after the addition of TNF-alpha; this suggests that the increased IL-10 mRNA level was due to de novo mRNA synthesis rather than mRNA stabilization; this latter finding was corroborated by actinomycin-D time course studies, which showed that the half-life of IL-10 was less than 1 h and was not significantly altered by TNF-alpha. These studies concerning IL-10 mRNA induction by TNF-alpha were corroborated by studies of IL-10 protein secretion: TNF-alpha alone, but not IL-1 alpha, IL-1 beta, or IL-6 induces substantial IL-10 secretion. Furthermore, LPS induces a large amount of IL-10 secretion that is largely inhibited (50 to 75%) by anti-TNF-alpha but not by antibodies to other inflammatory cytokines. Finally, TNF-alpha augments LPS-induced IL-10 secretion. Taken together, these findings suggest that TNF-alpha is unique among the inflammatory cytokines in its role as an inducer of IL-10 in human monocytes, as such, it induces a molecule that provides negative feedback to its own production.