A total of 74 clinical isolates of Branhamella catarrhalis were characterized with respect to their ampicillin, amoxicillin-clavulanate, cephalothin, cefaclor, erythromycin, tetracycline, chloramphenicol, and trimethoprim-sulfamethoxazole MICs and zones of inhibition. Disk diffusion tests were performed according to the guidelines of the National Committee for Clinical Laboratory Standards with two different media (Mueller-Hinton agar and chocolate Mueller-Hinton agar) and plates incubated under two atmospheric conditions (ambient air and 5 to 7% CO2). Optimum disk diffusion test results were obtained with Mueller-Hinton agar plates incubated in ambient air with all eight antimicrobial agents. On the basis of comparisons of MICs versus zones of inhibition, the following zone diameter interpretive criteria were defined for testing B. catarrhalis with disks containing 10 micrograms of ampicillin: greater than or equal to 38 mm, susceptible; 20 to 37 mm, moderately susceptible; less than or equal to 19 mm, resistant. The respective MIC correlates were less than or equal to 0.06, 0.125 to 0.5, and greater than or equal to 1.0 micrograms/ml. Because of the absence of frankly resistant test organisms, it was not possible to make definitive recommendations pertaining to disk diffusion tests with amoxicillin-clavulanate, cephalothin, cefaclor, erythromycin, tetracycline, chloramphenicol, and trimethoprim-sulfamethoxazole. Evidence is presented, however, which suggests that the current National Committee for Clinical Laboratory Standards disk diffusion interpretive criteria for nonfastidious bacteria can be applied to B. catarrhalis, at least as they pertain to the susceptible category with cephalothin, amoxicillin-clavulanate, erythromycin, tetracycline, chloramphenicol, and trimethoprim-sulfamethoxazole. With cefaclor, a zone diameter of greater than or equal to 21 mm was determined to adequately define the susceptible category.