From November 1, 1997 to April 30, 1998, 726 Moraxella catarrhalis isolates and 1529 Haemophilus influenzae isolates were obtained from 34 medical centres throughout the United States. Rates of beta-lactamase production were 94.6% among M. catarrhalis and 31.1% among H. influenzae strains. Susceptibility rates of M. catarrhalis isolates to selected antimicrobial agents were greater than 99% for amoxycillin-clavulanate, cefixime, cefpodoxime, cefuroxime, cefaclor, loracarbef, clarithromycin, azithromycin, chloramphenicol and tetracycline, 97.8% for cefprozil, 50.4% for trimethoprim-sulphamethoxazole and 28.1% for ampicillin. Of the antimicrobials tested against H. influenzae, the only agents with susceptibility rates below 96% were loracarbef (87.6%), cefprozil (83.4%), cefaclor (82.7%), trimethoprim-sulphamethoxazole (67.3%) and ampicillin (64.7%). The clarithromycin susceptibility rate was 67.4% but this agent was not tested in the presence of its 14-OH metabolite.