Between 1 August 1993 and 31 July 1994, 135 clinical isolates of Moraxella catarrhalis were collected from 12 large medical laboratories in Taiwan. The majority of specimens came from sputum (124 isolates). Other specimens included four isolates from throats, three isolates from wounds or pus, two isolates from eyes, one isolate from blood and one from cerebrospinal fluid. Epidemiologically, M. catarrhalis isolates were found frequently in winter and spring with a peak in February, and only sporadically from April to September. The overall rate of beta-lactamase producing isolates was 98.5% (132/135). All isolates were considered to be ampicillin-resistant, none were found to be resistant to other beta-lactam agents. Among other antimicrobial agents, all isolates were susceptible to chloramphenicol, erythromycin, roxithromycin, ofloxacin and ciprofloxacin, but uniformly resistant to trimethoprim (minimum inhibitory concentration (MIC) > or = 4 micrograms/mL, zone diameter < or = 19 mm). There were 12 isolates (8.8%) resistant to sulfamethoxazole (MIC > or = 32 micrograms/mL, zone diameter < or = 19 mm) and 19 isolates (14.4%) resistant to tetracycline (MIC > or = 16 micrograms/mL, zone diameter < or = 19 mm). The high level of resistance to ampicillin due to beta-lactamase production indicates that this is no longer a reliable agent for the treatment of M. catarrhalis infections. Among the beta-lactam agents tested, amoxicillin + clavulanate and the cephalosporins were active. These agents appear to be reliable first-line therapies when infection with M. catarrhalis is suspected. Misidentification of the species and difficulties in determining susceptibility to ampicillin are still widespread in Taiwanese laboratories. The application of the butyrate hydrolysis test and an appropriate test for beta-lactamase production is necessary for the resolution of these problems.