Fc epsilon receptor (Fc epsilon R) expression on several human cell lines (U937, RPMI 8866, HL 60, THP-1, and Molt 4) and its regulation were examined by immunofluorescent analysis using a monoclonal anti-human Fc epsilon R antibody, H107. Phorbol ester (PMA), recombinant gamma interferon (IFN-gamma) and H107 itself enhanced Fc epsilon R expression on a FC epsilon R positive cell line U937, whereas these reagents did not induce FC epsilon R expression on the Fc epsilon R negative cell lines, Molt 4, HL 60 and THP-1. Dexamethasone not only suppressed by 50% the spontaneous Fc epsilon R expression on U937 cells but also completely inhibited the enhancement of their Fc epsilon R expression on U937 cells induced by PMA, IFN-gamma or H107. Dexamethasone caused a little suppression of Fc epsilon R expression by RPMI 8866 cells. The results showed that Fc epsilon R expression on a human monoblast cell line U937 was up- or down-regulated by a variety of physiological or pharmacological agents. These experimental systems provide a good model for the investigation of the regulatory mechanisms of Fc epsilon R expression.