Interleukin-10 (IL-10) has been reported to be a negative cytokine for monocytes/macrophages. In the present study, we showed that IL-10 is rather a positive cytokine and augments the growth and differentiation of human monocytes stimulated with macrophage colony-stimulating factor (M-CSF ). Highly purified adherent human monocytes were cultured for 7 days with M-CSF in the presence or absence of IL-10. The number of recovered cells increased in the culture of monocytes with M-CSF + IL-10 compared to the culture with M-CSF alone. IL-10 alone was not enough to maintain the survival and differentiation of monocytes into macrophages. Morphological change cultured in M-CSF was also accelerated by addition of IL-10, and macrophages cultured in M-CSF + IL-10 were more elongated compared to macrophages cultured with M-CSF alone. Binding of ¹²⁵I-M-CSF to monocytes incubated with M-CSF + IL-10 was about 1.7-fold higher than that to monocytes incubated with M-CSF alone. In accordance with the binding study, Northern blot analysis showed that the levels of the expression of c-fms, M-CSF receptor, mRNA in macrophages cultured in M-CSF + IL-10 were higher than that in macrophages cultured in M-CSF alone. Macrophages cultured in M-CSF + IL-10 expressed higher level of FcγRI, II, III, and showed augmented Fcγ receptor mediated phagocytosis. The former also produced higher level of H₂O₂ and O⁻₂ , when stimulated with zymosan, and of IL-6 when stimulated with lipopolysaccharide compared to the latter. These results taken together suggest that IL-10 augments the growth and differentiation of human monocytes cultured in M-CSF.