We have evaluated the effect of the immunosuppressant sodium fusidate (fusidin) on the course of acute monophasic experimental encephalomyelitis (EAE) in male Lewis rats. Prophylactic treatment with fusidin, 80 or 120 mg/kg bd wt., markedly ameliorated the course of the disease in rats immunized with myelin basic proteins in complete Freund's adjuvant, entailing delayed onset of symptoms, lower clinical scores and more rapid recovery than PBS-treated control rats. The fusidin-treated, immunized rats exhibited milder mononuclear cell infiltration of brains and spinal cords than control animals. These data provide further evidence for the anti-inflammatory effect of fusidin and suggest that this drug may be valuable for the treatment of human multiple sclerosis.