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Among 53 Acinetobacter baumannii isolates collected in 2004, nine imipenem-resistant isolates were obtained from clinical specimens taken from patients hospitalized in Busan, Korea. Nine carbapenemase-producing isolates were further investigated in order to determine the mechanisms underlying resistance. These isolates were then analyzed via antibiotic susceptibility testing, microbiological tests of carbapenemase activity, pI determination, transconjugation test, enterobacterial repetitive consensus (ERIC)-PCR, and DNA sequencing. One outbreak involved seven cases of infection by A. baumannii producing OXA-23 beta-lactamase, and was found to have been caused by a single ERIC-PCR clone. During the study period, the other outbreak involved two cases of infection by A. baumannii producing IMP-1 beta-lactamase. The two clones, one from each of the outbreaks, were characterized via a modified cloverleaf synergy test and an EDTA-disk synergy test. The isoelectric focusing of the crude bacterial extracts detected nitrocefin-positive bands with pI values of 6.65 (OXA-23) and 9.0 (IMP-1). The PCR amplification and characterization of the amplicons via direct sequencing showed that the clonal isolates harbored blaIMP-1 or blaOXA-23 determinants. The two clones were characterized by a multidrug resistance phenotype that remained unaltered throughout the outbreak. This resistance encompassed penicillins, extended-spectrum cephalosporins, carbapenems, monobactams, and aminoglycosides. These results appear to show that the imipenem resistance observed among nine Korean A. baumannii isolates could be attributed to the spread of an IMP-1- or OXA-23-producing clone. Our microbiological test of carbapenemase activity is a simple method for the screening of clinical isolates producing class D carbapenemase and/or class B metallo-beta-lactamase, in order both to determine their clinical impact and to prevent further spread.