Josamycin (leucomycin A3) is a member of the leucomycin complex, a family of closely-related macrocyclic lactone antibiotics produced by Streptomyces kitasatoensis, discovered in 1953 by Omura and colleagues at The Kitasato Institute, Japan. Leucomycin complex is used as an animal health product for control of Gram-positive bacteria, Gram-negative cocci, leptospira and mycoplasma. Josamycin itself was not isolated until 1966. Josamycin inhibits influenza A virus infection and disease progression.