The effects of 1 alpha,25-dihydroxyvitamin D3/calcitriol on the expression of Fc receptors (FcR) for IgA (Fc alpha R), IgE (Fc epsilon RII), and IgG (Fc gamma R) on human peripheral blood monocytes and the cell lines U937, THP-1, and Mono Mac-6, in combination with various cytokines, was examined by flow cytometry. On both monocyte-derived macrophages and the myelomonocytic cell lines, Fc alpha R/CD89 expression was induced by calcitriol alone and additively in combination with tumor necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma), and granulocyte-macrophage colony-stimulating factor. Constitutive and interleukin-4 (IL-4)-driven Fc epsilon RII/CD23 expression was markedly diminished on calcitriol-treated U937 cells and monocytes. Fc epsilon RII was also triggered by IFN-gamma, TNF-alpha, and IL-6 on all the cell lines, an effect blocked by calcitriol. On monocytes, the basal level and IFN-gamma-induced Fc gamma RI/CD64 expression was down-regulated by calcitriol and IL-4. The expression of Fc gamma RII/CD32 on monocytes was strongly suppressed by calcitriol. Transforming growth factor-beta induced Fc gamma RIII/CD16 on monocytes, an effect opposed by calcitriol. The ability of calcitriol-treated monocytes to phagocytose IgG-sensitized ox erythrocytes was diminished. Our results demonstrate that calcitriol, alone or in combination with cytokines, modulates Fc alpha R, Fc epsilon RII, Fc gamma RI, and Fc gamma RII expression on human mononuclear phagocytes, as well as Fc gamma R-mediated phagocytosis.