Cells of a human monocyte-like cell line (U937) were analysed for IgE Fc receptors (Fc epsilon R) before and after glucocorticoid treatment. Specific binding of human myeloma IgE (Sha) was measured by 125I-labelled IgE, and by fluorescein-labelled IgE monitored by flow cytometry. Treatment of cells with dexamethasone or other steroids with glucocorticoid activity caused a significant decrease in Fc epsilon R expression. The inhibition was dose dependent, with a half-maximal effect at 20 nM dexamethasone, a concentration which is near to the dissociation constant of glucocorticoid receptors for dexamethasone. Inhibition of Fc epsilon R was significant beginning 8 h following glucocorticoid treatment and reached a plateau at 24 hr. The Ka for IgE binding was similar for control and dexamethasone-treated cells, while the number of IgE binding sites was decreased by 50-60%. Culture supernatants from dexamethasone-treated U937 cells which were concentrated 10-fold and depleted of free steroid did not affect Fc epsilon R expression. These results demonstrate that glucocorticoids can directly decrease the number of Fc epsilon R. This effect could participate in the glucocorticoid-induced suppression of IgE-mediated allergic reactions.