The in vitro activity of roxithromycin was compared with that of the other antimicrobial agents (erythromycin, tetracycline, ampicillin, and cotrimoxazole) against 188 distinct clinical isolates of Moraxella catarrhalis. Of these, 106 strains (66%) produced beta-lactamase. The MIC90 of roxithromycin was 0.25 micrograms/ml compared with values of 0.5, 1, greater than 8, greater than 8:0.4 micrograms/ml for erythromycin, tetracycline, ampicillin, and sulfamethoxazole-trimethoprim, respectively. These results, allied with its improved pharmacokinetic properties, suggest that roxithromycin should be an effective treatment in children and adults for upper and lower respiratory tract infections caused by M. catarrhalis.