Eight hundred and eighteen Moraxella catarrhalis strains, isolated in 1992 and 1993 at 15 centres in Western Europe and the USA were tested for beta-lactamase production and resistance to 15 antibiotics. The proportion of beta-lactamase producing strains in Europe rose significantly from 70% in 1992 to 82% in 1993, whilst in the USA the increase (85-92%) was not significant. Penicillin and amoxycillin resistance was more prevalent in the USA than in Europe. All penicillin-resistant strains isolated in the USA exhibited beta-lactamase activity, whilst 8% of beta-lactamase-negative strains isolated in Europe were also penicillin resistant. All but three isolates were sensitive to cefaclor, cefuroxime, cefixime and ceftriaxone, all but one were sensitive to ofloxacin and all were sensitive to ciprofloxacin and amoxycillin cluvulanate. Resistance to erythromycin was not encountered, although 19 strains had MICs > or = 0.5 mg/L. Of these, 15 came from European centres. Almost all strains were highly susceptible to clarithromycin, azithromycin, doxycycline and co-trimoxazole.