Acinetobacter baumannii strain A148, a clinical isolate resistant to imipenem (MIC=32 mg l −1 ), synthesized two β-lactamases with p I s 6.3 and >9.2. The p I 6.3 enzyme hydrolyzed the penicillins, including isoxazoylpenicillins, first-, second- and, to a lesser extent, third-generation cephalosporins. It was inhibited by chloride ions and by the penem β-lactamase inhibitor BRL 42715. Clavulanate was a weak inhibitor and EDTA did not affect the β-lactamase activity. This enzyme also hydrolyzed imipenem with a catalytic efficiency ( k cat / K m ) of 1500 mM −1 s −1 . Moreover, this purified β-lactamase produced a positive microbiological clover-leaf test with imipenem. Therefore, the p I 6.3 β-lactamase was considered to be involved in the imipenem resistance of A. baumannii strain A148.