Three carbapenem-resistant Acinetobacter baumannii isolates were collected at a hospital in Buenos Aires, Argentina. Isoelectric focusing revealed multiple beta-lactamases, with two of the isolates showing identical profiles. A pI 6.9 carbapenemase with a molecular weight of 30 kDa was purified from one of these two isolates. The enzyme was predominantly a penicillinase, with its highest Vmax for oxacillin but highest Vmax/Km for benzylpenicillin. First-generation cephalosporins and imipenem were weaker substrates than penicillins, and oxyimino-aminothiazolyl cephalosporins were essentially stable. Meropenem-hydrolysing activity was not detected, despite resistance. The carbapenemase was inhibited by clavulanic acid and tazobactam, but not by EDTA. These kinetics place the enzyme into functional group 2; as an oxacillinase it could be placed in sub-group 2d or, as a zinc-independent carbapenemase, in sub-group 2f.