Leinamycin (DC 107) is newly discovered antitumor antibiotic with an unusual 1,3-dioxo-1,2-dithiolane structure. Five different producing strains were isolated from soils collected in Japan during 1985-1988 and were taxonomically assigned as Streptomyces. Fermentation studies indicate: Leinamycin was unstable in culture broth. A chemically defined medium could be designed for a preferable production. Streptomyces sp. S-140 grew on medium supplemented with Zn2+ and high porus polymer resin and accumulated 32 micrograms/ml of leinamycin. Improved isolation methods are described along with identification of mikamycin A co-produced with leinamycin by the strain S-140.