We have very recently reported that a metabolite of Isaria Sinclairii (ATCC24400), identical with myriocin (=thermozymocidin) produced by Myriococcum albomyces (ATCC16425) and Mycelia sterilia (ATCC20349), showed potent immunosuppressive activity with 10 times and 100 times the activities of cyclosporin A (CsA) in mouse allogeneic mixed lymphocyte and cytotoxic T lymphocyte assays, respectively. Reduction of the ketone at C-14 of myriocin to a methylene increased the activity 10-fold, leading to the speculation that even more active immunosuppressants might be found in the culture broth of myriocin-producing microorganisms such as I. sinclairii, M. sterilia, and M. albomyces. Among these strains, M. sterilia was selected because it produced the greatest amount of myriocin and the minor components (mycestericins). Mycestericins A, B, C, D and E were isolated from the culture broth of Mycelia sterilia ATCC 20349 along with thermozymocidin (=myriocin). Their structures were elucidated on the basis of spectroscopic studies and chemical evidence, and the acetate of mycestericin C was identical with the acetate of 6,7-dihydromyriocin. Mycestericins suppressed the proliferation of lymphocytes. The present paper describes the isolation, structural determination and biological activities of mycestericins isolated from the culture broth of M. sterilia.