BACKGROUND: beta-lactam antibiotics are one of the most frequently used antimicrobial agents. However, with the increase of beta-lactamase-producing bacteria, penicillins arid 1 st generation cephalosporins have become less useful. Cefatrizine and clavulanic acid combination (CTCA) was developed to restore the activity. The aim of this study was to determine the activities of CTCA against major recent clinical isolates. METHODS: Aerobic and anaerobic bacteria tested were isolated from clinical specimens in Severance Hospital during 1996 to 1999. Antimicrobial susceptibility was determined by the NCCLS agar dilution methods. RESULTS: MICs of cefatrizine (CT) and CTCA were similar for methicillin-susceptible Staphylococcus aureus, Streptococcus pyogenes and S. pneumoniae. For Moraxella (Branhamella) catarrhalis, MIC90 CTCA was 1 microgram/mL, which was 1/8-fold lower than that of cefatrizine. MIC90S of CTCA for Escherichia coli and Klebsiella pneumoniae were 4 microgram/mL and 8 microgram/mL, respectively, which were 1/4- to 1/16-fold lower than those of CT. However, it was less active against Citrobacter freundii, Enterobacter cloacae and Serratia marcescens. Against Bacteroides fragilis group organisms, it showed good activities similar to those of other beta-lactam and beta-lactamase inhibitor combinations. CONCLUSIONS: CTCA showed good antimicrobial activities against M. (B.) catarrhalis, Haemophilus influenzae, Neisseria gonorrhoeae, extended spectrum beta-lactamase-producing E. coli and K. pneumoniae, Proteus vulgaris and B. fragilis. In conclusion, it would be useful for the treatment of infections due to those organisms, and for the empirical treatment of respiratory and urinary tract infections.