We previously described a reduction of silica-induced lung fibrosis in interleukin-10-deficient mice (IL-10−/−) (Huaux and colleagues; Am. J. Respir. Cell Mol. Biol. 1998;18:51–59). In the present study, we further dissect the exact functions of IL-10 in experimental silicosis. The reduced lung fibrotic response to silica in IL-10−/− mice was accompanied by a marked recruitment of TH1 CD4+ lymphocytes. However, treatment with anti-CD4 antibodies reduced silica-induced lung fibrosis in both IL-10−/− and IL-10+/+ mice, suggesting that this T cell population actually contributes to the extension of the fibrotic lesions in a manner that is independent of IL-10. In IL-10−/− mice, silica-induced lung production of the profibrotic mediator transforming growth factor (TGF)-β1 and the antifibrotic eicosanoid PGE2 were reduced and increased, respectively, relative to that in IL-10+/+ mice. In addition, in vitro experiments indicated that recombinant IL-10 upregulated TGF-β1 expression in alveolar macrophages while in contrast it downregulated PGE2 production and cyclooxygenase-2 expression in both lung fibroblasts and macrophages. Thus the net profibrotic activity of IL-10 in vivo appears to be mediated by its ability to stimulate the expression of the profibrotic cytokine TGF-β1 while suppressing the expression of cyclooxygenase-2 and thus production of the antifibrotic eicosanoid PGE2. These effects appear to be independent of the enhanced lung CD4+ T-lymphocytosis observed in IL-10-deficient mice.