Tacrolimus (FK506) is a new macrolide immunosuppressant isolated from Streptomyces tsukubaenesis that acts by a variety of different mechanisms, including inhibition of calcineurin. Although the mechanism of action of tacrolimus is similar to that of ciclosporin A, tacrolimus is 10 to 100 times more potent. Tacrolimus also has an effect on glucocorticoid receptor-mediated gene expression. The therapeutic efficacy of tacrolimus in primary immunosuppression has been evaluated in numerous clinical trials in patients receiving hepatic, renal, heart, lung, pancreas, intestinal, or bone marrow transplantation. Because tacrolimus displays a variety of different mechanisms of action and shows a synergism with corticosteroids, it is presumed that tacrolimus provides a new and useful therapeutic approach for autoimmune diseases, including myasthenia gravis.