Fifty-four clinical isolates of Branhamella catarrhalis from patients with bronchopulmonary infections were studied. The MICs for 50 and 90% of the isolates and the geometric mean MICs were determined for 11 antimicrobial agents. All the strains were resistant to trimethoprim but were susceptible to clavulanate-potentiated amoxicillin (Augmentin; Beecham Research Laboratories, London), chloramphenicol, co-trimoxazole, erythromycin, cefotaxime, and cefuroxime. Beta-lactamase-negative strains were uniformly susceptible to penicillin and ampicillin.