A new antibiotic designated as SF-1623 substance exhibiting a useful antibacterial activity against gram-negative bacteria is now produced by cultivating a new strain ATCC 21999 of the known species Streptomyces chartreusis. The SF-1623 substance in the form of its an alkali metal salt may be recovered from the culture when the cultivation is effected under aerobic conditions in the presence of an alkali metal thiosulfate.
