HRE 664, a new penem antibiotic, inhibited 90% of Escherichia coli, Klebsiella, Citrobacter diversus, Proteus mirabilis, Proteus vulgaris, Salmonella, Shigella, Providencia, Aeromonas, and Morganella at less than or equal to 2 micrograms/ml but was considerably less active than cefotaxime, ceftazidime, and imipenem. It did not inhibit Pseudomonas aeruginosa (MIC greater than 128 micrograms/ml). HRE 664 inhibited Enterobacter spp., Citrobacter freundii, and Serratia marcescens at 1-8 micrograms/ml, two- to fourfold higher MICs than imipenem. HRE 664 inhibited methicillin-susceptible Staphylococcus aureus and Staphylococcus epidermidis at less than or equal to 0.12 micrograms/ml, but methicillin-resistant S. aureus and S. epidermidis were resistant. Group A, C, and G streptococci and Streptococcus pneumoniae were inhibited by 0.06 micrograms/ml. Bacteroides and Clostridium species were inhibited by 0.25 micrograms/ml comparable to imipenem. HRE 664 was not hydrolyzed by beta-lactamases TEM-1, TEM-2, TEM-3, TEM-5, SHV-1, PSE-1, PSE-4, OXA-2, OXA-3, K-1, P99, Morganella, P. vulgaris, and S. aureus PC-1 but was hydrolyzed by the beta-lactamase of Xanthomonas maltophilia.