The expression of Fc receptors for IgG (Fc gamma R) and IgA (Fc alpha R) and of various other antigens on the human monocytic cell line U937 and peripheral blood monocytes, under stimulation with human recombinant tumour necrosis factor-alpha (TNF-alpha) and other cytokines, was investigated by flow cytometry. TNF-alpha, as well as interferon-gamma (IFN-gamma) or interleukin-6 (IL-6) had a significant up-regulating effect on U937 expression of Fc gamma RI/CD64. Furthermore, the action of TNF-alpha was augmented by IL-6, and more evidently by IFN-gamma. IFN-alpha alone had only a marginal effect, but was able to increase the TNF-alpha-driven Fc gamma RI expression. In contrast to U937 cells, TNF-alpha did not enhance significantly Fc gamma RI expression on human monocytes. Interestingly, on both U937 cells and monocytes, Fc alpha R was augmented markedly by TNF-alpha. Furthermore, TNF-alpha induced the expression of HLA-DR and HLA-DP antigens on monocytes and U937 cells. The expression of Fc gamma RII/CD32, FC gamma RIII/CD16, CD14, complement receptor type 1 (CR1/CD35), CR4 (CD11c/CD18), and MHC class-I antigens, was not influenced significantly by TNF-alpha. The results of this study show that TNF-alpha may act on human mononuclear phagocytes, alone or in combination with other cytokines, by modulating the expression of various cell-surface antigens.