Antibiotic PS-5l) is a new β-lactam antibiotic isolated from fermentation broths of Streptomyces sp. strain A271. The strain was considered to be a new subspecies of Streptomyces cremeus and the name, Streptomyces cremeus subsp. auratilis, was proposed. Fermentative production, isolation and physico-chemical properties of PS-5 are described. During the course of screening for antibiotics produced by strains of Actinomycetes, several strains of Streptomyces were found to produce a new β-lactam antibiotic possessing strong antimicrobial activity against organisms resistant to known β-lactam antibiotics2) and also having inhibitory activity against β-lactamases from Bacillus cereus, Bacillus licheniformis3) and Proteus vulgarisj4). One of the producing organisms, strain A271, was identified as a new subspecies of Streptomyces cremeus, and was named Streptomyces cremeus subsp. auratilis. In this paper, taxonomic studies of strain A271, fermentative production, isolation and physico-chemical properties of PS-5 are reported. © 1979, JAPAN ANTIBIOTICS RESEARCH ASSOCIATION. All rights reserved.