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Branhamella catarrhalis obtained from the sputum of 146 patients with lower respiratory tract disease and from middle ear fluids of 26 children with otitis media were evaluated for beta-lactamase activity and the enzymes were characterised by isoelectric focusing (IEF). 71% (103 of 146) of the sputum isolates and 77% (20 of 26) of the ear isolates produced beta-lactamase. By IEF, the beta-lactamases of 113 of 123 (92%) strains revealed patterns identical with the Ravasio type strain, having unique enzyme bands at pIs of 6.4 and 6.65. The remaining 10 isolates (8%) produced patterns similar to the 1908 type strain with a unique band of activity having a pI of 6.55. In addition, the 1908 types revealed a band of minor enzyme activity with a pI of 7.55 that was absent from the Ravasio types. All strains tested shared major enzyme bands with pIs of 5.1, 5.3, 5.55 and 6.1. These results indicate that the most common beta-lactamase(s) produced by clinical isolates of B. catarrhalis in the United States are similar to those produced by the Belgian Ravasio type strain.