Four isolates of Klebsiella pneumoniae obtained from patients at a Maryland medical centre exhibited reduced susceptibility to carbapenems and were found to produce the novel, class A, plasmid-mediated, carbapenem-hydrolysing enzyme, KPC-2. This enzyme has 99% identity with the plasmid-mediated, carbapenem-hydrolysing enzyme KPC-1, reported previously in a North Carolina K. pneumoniae isolate. The KPC-2-producing isolates were either susceptible or intermediate to imipenem and meropenem, unlike the KPC-1-producing isolate, which was resistant to these agents. Detection of KPC-2 may be a problem for clinical laboratories because in this study it was associated with positive extended-spectrum beta-lactamase (ESBL) confirmation tests (clavulanate-potentiated activities of ceftriaxone, ceftazidime, cefepime and aztreonam). Therefore, a failure to recognize the significance of reduced carbapenem susceptibility in the isolates that remained susceptible to imipenem or meropenem could have resulted in the isolates being incorrectly identified as ESBL producers.