Concanamycins A, B and C were isolated from the mycelium of Streptomyces diastatochromogenes S-45 as effective inhibitors of the proliferation of mouse splenic lymphocytes stimulated by concanavalin A. They represent a new class of 18-membered macrolide antibiotics, and are biologically active in vitro against several fungi and yeasts, but not against bacteria. Concanamycin A, the main component, has been identified with antifungal antibiotics, folimycin and A-661-I.