BACKGROUND: Extended-spectrum beta-lactamases (ESBLs) are plasmid-mediated enzymes that confer resistance to oxyimino-beta-lactams such as cefotaxime, ceftazidime, and aztreonam, antibiotics that were designed to be effective against strains producing known plasmid-determined beta-lactamases. METHODS: Fifty-seven isolates of ESBL producing K. pneumoniae which were collected from 3 hospitals in Korea were characterized. They either transferred ceftazidime resistance or showed clavulanic acid enhancement of oxyimino-beta-lactam susceptibility. Beta-lactamase production by clinical isolates and transconjugants were characterized by isoelectric focusing and MIC values of oxyimino-beta-lactam were determined by agar dilution method. We performed PCR and RFLP for further characterization of TEM-type ESBLs. RESULTS: The results showed that SHV-type ESBLs, especially SHV-5 and SHV-2 predominated, but TEM-4 and plasmid- mediated AmpC beta-lactamases (pI 8.0 and pI 8.4) were present as well: SHV-5, 26; SHV-2, 12; TEM-4, 9; plasmid-mediated AmpC type beta-lactamase (pI 8.0), 4; plasmid-mediated AmpC type beta-lactamase (pI 8.4), 1. Two isolates produced both TEM-4 and SHV-5, which showed higher MIC50 of cefotaxime, ceftazidime and aztreonam than those in the isolates producing TEM-4 or SHV-5. Three isolates produced both plasmid-mediated AmpC type beta-lactamases and other ESBLs, each one of TEM-4, SHV-2 and SHV-5. All of the isolates showed high resistance to cefoxitin, cefotaxime, ceftazidime and aztreonam. CONCLUSION: With these results we conclude that SHV-type ESBLs outnumber and plasmid-mediated AmpC type beta-lactamases are present in high frequency in Korea.