Over six months Branhamella catarrhalis was isolated in pure culture from the sputum of 81 patients with symptoms of acute respiratory tract infection. Of 38 patients who were infected in the community, over half required admission to hospital. The remaining 43 patients acquired the infection in hospital. Forty one of the 81 isolates produced beta-lactamase, 24 of these being hospital acquired infections. As a result 40% of patients who were treated with ampicillin did not respond. Most patients had chronic lung diseases or lung cancer or were taking corticosteroids. Three patients died and one required assisted ventilation; strains producing beta-lactamase were isolated in each case. Acute bronchitis developed in one previously healthy young non-smoker. It is concluded that B catarrhalis is an important pathogen of the lower respiratory tract which should be reported, and strains producing beta-lactamase should be identified. Otherwise, treatment with inappropriate antibiotics may result in increased morbidity or mortality.