B. catarrhalis is a potential pathogen in the upper and lower respiratory tract which has been implicated as a clinically important cause of chronic bronchitis and otitis media in children. Since the late 1970s the proportion of B. catarrhalis strains elaborating beta-lactamase seems to have significantly increased; some centres are now reporting prevalence rates as high as 76%. Such a dramatic increase in the number of beta-lactamase positive strains is of clinical importance when assessing the indirect pathogenic potential of B. catarrhalis and when selecting suitable antimicrobial therapy. Early studies showed that B. catarrhalis was sensitive to penicillin V with a MIC90 of 1.2 mg/L while, more recently, MIC90 values of 2.0 mg/L have been noted. Ampicillin and, perhaps surprisingly, cefaclor are also inactivated by some beta-lactamase-producing strains of B. catarrhalis. A majority of strains of B. catarrhalis is susceptible to erythromycin (MIC90 0.15 to 0.5 mg/L) and tetracyclines (especially doxycycline, MIC90 0.25 to to 0.5 mg/L). Co-trimoxazole also seems to be effective against most isolates of B. catarrhalis whereas trimethoprim alone is relatively ineffective.