PS-5, a new beta-lactam antibiotic, has relatively potent antimicrobial activity against Gram-positive and Gram-negative bacteria, especially the Enterobacter groups, Serratia marcescens, the Proteus groups and Klebsiella pneumoniae. The activity of PS-5 against many beta-lactamase-producing organisms is greater than that of cefoxitin or cefazolin. PS-5 has good therapeutic activity in mice infected with Staphylococcus aureus Smith or Enterobacter cloacae 45.