We tested 231 isolates of BranhameUa catarrhalis for P-lactamase production and drug susceptibility by the National Committee for Clinical Laboratory Standards disk diffusion method. The nitrocephin disk (Cefinase) identified 13-lactamase in 98% of the enzyme-producing strains, and a zone diameter of inhibition of <29 mm for penicillin correctly predicted the presence of P-lactamase in 99% of the isolates. No resistance to erythromycin, tetracycline, trimethoprim-sulfamethoxazole, or amoxicillin-clavulanic acid was observed.Branhamella catarrhalis has received increasing attention as a potential pathogen of the upper and lower respiratory tract (4). Otitis media, sinusitis, bronchitis, and bronchopneumonia (the latter two primarily in patients with underlying lung disease) appear to be the major clinical diseases (9,17,18,21). In a recent study with Gram stain-directed cultures, B. catarrhalis was recovered from 5% of all acceptable sputum samples from patients with chronic lung disease over a 30-month period, and 11.5% of the samples contained a recognized pathogen; this latter incidence approached 20% in the winter months (16).Of major concern therapeutically has been the recognition that B. catarrhalis produces P-lactamase. The first known report of P-lactamase in this species appeared in 1977 (10).Since that time 3-lactamase production has spread, and many isolates now produce the enzyme (2,14,20 Cockeysville, Md.) and by the addition of extracts from acetone-treated cells (11) to a solution of nitrocephin (15).Disk diffusion susceptibility testing was done at the time of isolation with MHA by the procedure recommended by the National Committee for Clinical Laboratory Standards (NCCLS) (12). Isolates which failed to grow adequately were retested under 5% CO2 with chocolatized MHA. The majority of isolates were frozen at -70°C and regrown at a later date for the P-lactamase testing of cell extracts and for isoelectric focusing (IEF).Crude extracts from acetone-treated cells containing 1B-lactamase were submitted to IEF on polyacrylamide gels with LKB instruments. The details of this procedure and IEF results for many of the current isolates have been presented elsewhere (11).A total of 231 isolates were identified during the study period. With the Cefinase disk, 171 isolates (74%) were 13-lactamase positive. Of the 231 isolates, 146 were also tested for ,B-lactamase production by using cell extracts. Two isolates which were 1-lactamase negative by the disk test were positive with the acetone-treated cell extracts. Both isolates which gave an initial negative ,B-lactamase result had the 1908-type enzyme.Twenty-six isolates (11%) failed to grow adequately on MHA in room air and had to be restudied in 5% CO2 on chocolatized MHA. The other 89% produced good growth on overnight incubation on MHA. By disk diffusion, no isolates were identified which were resistant to erythromycin, moxalactam, amoxicillin-clavulanic acid, chloramphenicol, cefotaxime, trimethoprim-sulfamethoxazole, or sulfisoxazole. There was no diff...