Moraxella catarrhalis is an important pathogen in both upper and lower respiratory tract infections. More than 90% of isolates worldwide produce beta-lactamase. The beta-lactamases produced by M. catarrhalis can be differentiated by isoelectric focusing (IEF) into BRO-1 and BRO-2 patterns. In this study, we investigated the prevalence of various beta-lactamase isoforms in clinical isolates of M. catarrhalis in Taiwan, as well as the relationships of these isoforms with antibiotic resistance. A total of 271 clinical isolates of M. catarrhalis were collected from 12 large medical laboratories in Taiwan from 1 August 1993 to 31 July 1995. The overall prevalence of beta-lactamase production was 98.2% (266 of 271 isolates). Analytical IEF revealed BRO-1 was the most common beta-lactamase pattern among the isolates (238 isolates, 88%); BRO-2 was the only other pattern found, with 32 (12%) isolates. The geometric mean minimum inhibitory concentration of ampicillin for BRO-1 producers was 63-fold higher than that for beta-lactamase-negative isolates, and 6.5-fold higher than that for BRO-2 producers. beta-Lactam antibiotics, such as amoxicillin + clavulanate and the cephalosporins, tested were very active against this species, regardless of whether the isolate produced beta-lactamase or not. In conclusion, beta-lactamase is common among clinical isolates of M. catarrhalis in Taiwan, with BRO-1 being the most common isoform. However, because most isolates tested were still sensitive to amoxicillin + clavulanate and cephalosporins, these agents appear to be reliable alternatives to first-line therapy when M. catarrhalis is contributing to a clinical infection.