Monocytes stimulated with bacterial lipopolysaccharide (LPS) generate a procoagulant activity (PCA) related to the induction of tissue factor (TF) expression at their surface. Since interleukin-10 (IL-10) was recently shown to inhibit LPS-induced cytokine production and is currently considered as a potential therapeutic agent in septic shock, we were interested to determine its effects on LPS-induced monocyte PCA. Peripheral blood mononuclear cells (PBMC) from healthy donors were incubated with 1 microgram/ml LPS in the presence of serial dilutions of recombinant human IL-10 and PCA was determined after 6 h in a one-stage clotting assay. IL-10 inhibited in a dose-dependent manner LPS-induced TF-dependent PCA: a significant effect was already observed with 30 pg/ml IL-10 while 64-97% inhibition was achieved with 120 pg/ml IL-10. In parallel flow cytometry experiments, IL-10 was shown to block LPS-induced TF expression at the surface of monocytes. In order to inhibit LPS-induced PCA, IL-10 had to be added to PBMC at least 6 h before LPS challenge. This inhibitory effect of IL-10 was already apparent at the TF mRNA level and was prevented by co-incubation with cycloheximide (20 micrograms/ml). These data suggest that IL-10 acts via the induction of protein(s) which might interfere with TF gene transcription or mRNA stability. We conclude that the protective effects of IL-10 in endotoxinemia might be related not only to cytokine synthesis blockade but also to inhibition of LPS-induced PCA.