BACKGROUND: Extended-spectrum beta-lactamases (ESBLs) confer resistance to extended-spectrum cephalosporin (e.g., cefotaxime, ceftazidime) and aztreonam. But the diversity of ESBLs results in various susceptibility profiles with different beta-lactams. To study the relative in vitro activities of various beta-lactams and non-beta-lactam antibiotics against the clinical isolates of ESBL-producing K. pneumoniae, we determined the MIC (minimum inhibitory concentration) of various antimicrobials. METHODS: Fifty-seven isolates of K. pneumoniae which produced ESBL and 63 isolates which did not produce ESBL from 3 university hospitals in Korea were tested. The MIC values of antimicrobials were determined by agar dilution method and detection of ESBL production was performed by double disk synergy test. RESULTS: The MIC values of beta-lactams against K. pneumoniae which produced ESBLs exhibited heterogeneous susceptability profiles. In differentiation of ESBL production, MIC value of 8 ug/mL (breakpoint of intermediate resistance) of ceftazidime was more sensitive and more specific than that of cefotaxime or aztreonam. MIC50 values of gentamicin, amikacin and ciprofloxacin against K. pneumoniae that produced ESBL were significantly higher than those against Non-ESBL producing isolates (P<0.001), suggesting that ESBL producing isolates are multi-drug resistant. CONCLUSION: The level of resistance to various beta-lactams of K. pneumoniae which produced ESBL was heterogeneous. ESBL-producing K. pneumoniae showed higher resistance to aminoglycoside and quinolone antibiotics. Ceftazidime was the most appropriate antibiotic to differentiate ESBL production.