Leucomycin, discovered in 1953, is an antibiotic complex of over fifteen closely related macrocyclic lactone analogues produced by Streptomyces kitasatoensis. The complex comprises four major components, A1, A4, A5 and A13, together with a number of poorly characterised minor analogues. The Leucomycin Complex Set contains the complex and its purified major components as standards for research.