Microbial products have amazing diversity and complexity in their chemical structures as well as in their biological activities. The successful discovery and development of FK506 (Tacrolimus) adds a new example showing that microbial products are agents with potential therapeutic benefits. In 1984, we found a novel immunosuppressant FK506 during the course of our research program for discovery of specific inhibitors on IL-2 production in the microbial products. FK506, a new class of 23-membered macrolide lactone, was isolated the fermentation broth of Streptomyces tsukubaensis No.9993. This agent strongly inhibited the proliferative response of lymphocytes to alloantigen stimulation, and a variety of T cell associated immune reaction. FK506 also suppressed immune responses in vivo as well as in vitro and this immunosuppressive effect was more highly potent than cyclosporin, a well-known fungal metabolite. The profound immunosuppressive properties of FK506 were subsequently explored in organ transplantation in animal models. Clinical trials of FK506 were begun in 1989 by Professor Stazl and his colleagues at the University of Pittsburgh. Since then wider evaluation of FK506 in a variety of transplantation fields has been performed globally; in North America, Europe and Japan. In 1993, FK506 was commercialized in Japan for prevention of liver transplant rejection. FK506 is also undergoing extensive evaluation as an agent for the treatment of various autoimmune diseases.