The in vitro activities of the carbapenem BMS-181139 were determined in comparison with those of imipenem, meropenem, ciprofloxacin, ceftriaxone, and vancomycin. BMS-181139 was the most active against species of Pseudomonas and related genera Alteromonas and Burkholderia, with MICs for 147 of 149 isolates of < 4 micrograms/ml. Of 22 imipenem-resistant (MIC > 8 micrograms/ml) P. aeruginosa strains, only 1 required an MIC of BMS-181139 of > 4 micrograms/ml, compared with 14 requiring the same meropenem MIC. BMS-181139 was the most active carbapenem against the majority of other gram-negative species except members of the tribe Proteeae, against which meropenem was more active. Although imipenem was more active against gram-positive species, BMS-18139 MICs at which 90% of strain tested were inhibited were < 1 microgram/ml for these species. BMS-181139 was generally active against isolates resistant to ciprofloxacin or broad-spectrum cephalosporins, including those containing plasmid-encoded beta-lactamases or high levels of chromosome-encoded beta-lactamases, as well as anaerobes except Clostridium difficile. Inoculum effects were noted for all three carbapenems against Klebsiella pneumoniae, Enterobacter cloacae, and Serratia marcescens but not Escherichia coli, Pseudomonas aeruginosa, or Staphylococcus aureus. BMS-181139's inoculum effect tended to be more marked. BMS-181139 exhibited bactericidal activity at the MIC for some strains and up to four to eight times the MIC for others. The postantibiotic effect of BMS-181139 was equal to or less than that of imipenem and, like meropenem, exhibited intraspecies variability. BMS-181139 was 30-fold more stable than imipenem and 7-fold more stable than meropenem to hydrolysis by hog kidney dehydropeptidase.