The in vitro activity of BO-2727, a new carbapenem, was compared with those of meropenem, biapenem, imipenem, and ceftazidime. BO-2727 was four- or eightfold more active than the other carbapenems against methicillin-resistant staphylococci and Pseudomonas aeruginosa strains, including imipenem- and ceftazidime-resistant bacteria. BO-2727 was quite stable to penicillinases, cephalosporinases, and oxyiminocephalosporinases, but not to metallo-beta-lactamase. Time-kill studies against Staphylococcus aureus Smith, Escherichia coli ML4707, and P. aeruginosa GN11189 showed that BO-2727 has potent bactericidal activity at concentrations greater than the MIC.