Siomycin, a sulfur-containing peptide antibiotic active against gram-positive bacteria and mycobacteria, has been isolated from cultures of Streptomyces sioyaensis. Further purification of siomycin preparations has shown that siomycin consists of one major (siomycin A) and two minor (siomycins B and C) compounds. Siomycins A, B and C were isolated as single components from crude siomycin preparations and characterized in detail. Their physicochemical properties are similar. Siomycin B is derived from siomycin A during storage, while siomycin C is a natural product of Streptomyces sioyaensis. The present paper deals in more detail with the physicochemical properties of siomycins A, B and C.