Inflammation in asthma and other allergic diseases is characterized by excessive production of immunoglobulin E (IgE) and the influx of leukocytes, especially eosinophils. Interleukin 4 (IL-4) and IL-5 are essential for IgE production and eosinophilia, respectively, and are produced by mast cells in allergic conditions, for which glucocorticoids are widely used therapeutically. We assessed the effect of glucocorticoids on IL-4 and IL-5 mRNA production by the RBL-2H3 cell line, an analog of mucosal mast cells. IL-4 and IL-5 mRNAs were induced by an antigen that is used to cross-link receptor bound IgE, by calcium ionophore, or by ionophore with phorbol ester and were markedly inhibited by dexamethasone. In cells activated with ionophore and phorbol ester, 10⁶ M dexamethasone reduced the IL-4 and IL-5 mRNA levels to only 12.8 and 5.7%, respectively, of those in cells without dexamethasone, and 10⁹ M dexamethasone caused reductions to 27 and 56%, respectively. Hydrocortisone at 10⁶ and 10⁷ M almost completely inhibited IL-4 and IL-5 mRNA production. Dexamethasone was markedly inhibitory even if it was added after the cells were activated, provided that it was present in the cultures for at least 1.5 h. These studies indicate that the expression of IL-4 and IL-5 mRNAs by mast cells is highly sensitive to glucocorticoids. The data suggest that these inhibitory effects may contribute to the clinical efficacy of glucocorticoids in the therapy of allergic diseases.