Bicyclomycin is a new antibiotic obtained from the culture filtrate of Streptomyces sp. WS4545 which was identified as a new species and given the name Streptomyces sapporonensis. Its elementary analysis and mass spectroscopic measurement suggest that the molecular formula is C12H18N2O7. There is no specific ultraviolet absorption. Bicyclomycin is active against Gramnegative bacteria, especially Escherichia colt, Klebsiella, Shigella and Salmonella species, and has no cross resistance with the usual antibiotics. It has low toxicity in mice. In the course of our antibiotics screening program, a Streptomyces initially designated WS4545 was found to produce an antibiotic with activity against Gramnegative bacteria. From its unique chemical and biological properties, it was judged to be a new antibiotic and named bicyclomycin. In this paper, the characteristics of strain WS4545, fermentation, isolation procedures, chemical and biological properties of bicyclomycin are described.