Cirramycin A, a macrolide antibiotic, has been found to consist of one major (A₁) and several minor components (A₂, A₃, A₄ and A₅). The major active substance, cirramycin A₁ (C₃₁H₅₁NO₁₀), was isolated as a single component and characterized in detail. It has a strong ultraviolet absorption peak at 241mμ and its chromophore is suggested to be an α,β-unsaturated T,depoxide carbonyl system. Cirramycin A₁ contains the basic sugar mycaminose but lacks a neutral sugar in the molecule. In the previous publication, cirramycin was reported to belong to the macrolide group of antibiotics and contain at least two components, A and B. Recent studies revealed that cirramycin A could be further separated into several closely related components, with the major one isolated in crystalline form and designated cirramycin A₁. This paper reports the production, isolation and characterization of cirramycin A₁.