Fusidic acid and its sodium salt (fusidin) are anti-staphylococcal drugs with a steroidal primary structure. Both compounds have been shown to prevent the lymphocyte co-stimulatory activities of the cytokines, interleukin (IL)-1 and IL-6, in a manner similar to that of cyclosporin A. As shown in this paper, fusidin also prevents the inhibitory effect of human recombinant IL-1 beta (rIL-1 beta) and the stimulatory effect of human rIL-6, on glucose-induced insulin production in vitro by normal rat pancreatic islets. The drug also inhibited rIL-1 beta-induced IL-6 production by the islets. Fusidin showed a dose-related effect at pharmacologically relevant concentrations from 3 to 30 micrograms/ml, and the drug was progressively less active when added 1, 4 and 24 h after rIL-1 beta. Electron microscopical studies showed that beta cells cultured for 72 h with rIL-1 beta accumulated less lipid in the presence of fusidin, most probably reflecting the functionally protective effect of the drug. Other characteristic ultrastructural changes induced in beta cells by rIL-1 beta were, however, not altered. It is suggested that fusidin may prove clinically effective as a modulator of IL-1- and IL-6-induced changes in beta-cell functions.