IL-10, originally described as a cytokine synthesis inhibitory factor, is secreted by a number of cells of the immune system, including monocytes and T cells. Although IL-10 is being assigned as an immunosuppressive cytokine, our study showed that FMLP-R mRNA was rapidly up-regulated by exposure of monocytes to graded concentrations of this cytokine, with maximal (three- to fourfold) stimulation with 10 ng/ml. The effect was rapid, being observable as early as 1 h of treatment with IL-10, maximal between 2 and 4 h, and still evident after 24 h and was associated with an increase of receptor expression on the cell surface as assessed by flow cytometry analysis. Pretreatment of monocytes with actinomycin D completely abrogated the effect of IL-10, suggesting a transcriptional regulation. Moreover, IL-10-treated monocytes showed a significantly enhanced functional responsiveness to FMLP with enhanced (three- to fourfold) chemotaxis and augmented (twofold) intracellular calcium mobilization. In polymorphonuclear neutrophils (PMN), IL-10 also mediated a twofold augmentation of FMLP-R expression. In parallel experiments, we observed that IL-10 could differentially modulate other chemotactic receptors. Hence, we observed that IL-10 augmented two-to threefold platelet-activating factor receptor (PAF-R) expression, whereas it had no significant effect on the fifth component of complement (C5a) receptor (C5a-R) expression. Collectively, our results demonstrate that IL-10 may play an important role in inflammatory process through modulation of chemotactic receptor expression.