Clinical isolates of Moraxella catarrhalis (76 isolates) were screened for beta-lactamase production and antibiotic susceptibility. beta-Lactamases (detected in 90.8% of isolates) were typed using isoelectric focusing to BRO-1 (87%) and BRO-2 (13%). Minor variations in electrofocusing patterns between the two types were seen. Isolates expressing BRO type enzymes showed solid resistance to penicillin, ampicillin and cephalothin, in particular BRO-1 producers. BRO-1 isolates were less susceptible to cephems and to beta-lactamase inhibitors than BRO-2 isolates. Isolates harbouring BRO-1 enzymes have more enzymatic activity than those expressed by BRO-2 isolates. Apart from resistance to tetracycline (14.5%), all isolates were consistently susceptible to erythromycin, chloramphenicol, ciprofloxacin and gentamicin. The conjugal transfer of BRO beta-lactamase gene(s) between M. catarrhalis isolates occurred with a frequency of 10(-5) to 10(-7)/donor cell. The data emphasize the importance of M. catarrhalis as an etiological agent spreading beta-lactamases that may inhibit some beta-lactams and lead to failure in treatment of mixed infections.