Interferon α (IFN-α) has proven its clinical usefulness in a variety of diseases of diverse pathogenesis. In addition to direct antiviral effects, recent evidence suggests that its interaction with the cytokine cascade might contribute to its mechanism of action. This study was undertaken to determine whether IFN-α influences the synthesis of interleukin 4 (IL-4) and IL-13, two cytokines which share many biological properties on various cells and tissues and which have a profound role in regulating immunological and inflammatory responses. Peripheral blood mononuclear cells (PBMC) from healthy volunteers were stimulated with Concanavalin A (ConA), phorbol myristate acetate (PMA), anti-CD3/CD28 mAbs, either alone or in various combinations, and incubated with increasing concentrations of IFN-α. IL-4 and IL-13 mRNA was determined by Northern hybridizations and IL-4 and IL-13 protein synthesis was evaluated by specific enzyme-linked immunosorbent assay (ELISA). IFN-α led to a profound decrease of IL-13 mRNA expression after an incubation period of 5 h with ConA alone or in combination with PMA, whereas it showed no regulatory effect on IL-4 mRNA expression. After an incubation period of 24 h, the decrease in IL-13 mRNA expression after addition of IFN-α was even more pronounced. At the protein level, IFN-α increased IL-4 synthesis dose dependently regardless of the mode of activation. This increase was most pronounced after stimulation with ConA or anti-CD28/PMA. In contrast, IL-13 synthesis was strongly downregulated by IFN-α in a dose-dependent manner irrespective of the activating agent. It is concluded that IL-4 and IL-13, although showing similar biological effects, are differentially regulated by IFN-α.