A group of 36 patients, admitted to hospital because of acute purulent exacerbations of chronic bronchitis, were treated with once daily injections of ceftriaxone for 10 days, 17 receiving 1 g injections and 19 patients 2 g doses. At the end of treatment (day 11) six patients remained infected (three with Branhamella catarrhalis and three with Pseudomonas aeruginosa) but during the 7 follow-up days 12 patients developed infections with beta-lactamase producing strains of Bran. catarrhalis, Ps. aeruginosa was cultured from 2 patients and Streptococcus pneumoniae from 3 more. Kinetic studies confirmed the long half-life of ceftriaxone (13 to 14 h in this patient group) and showed average peak serum concentrations of 31 mg/l after 1 g and 43 mg/l after the 2 g dose. The comparable sputum concentrations were 3.5 and 4.8 mg/l, respectively. However, four patients failed to show any ceftriaxone in the sputum despite simultaneous blood concentrations of between 32 and 50 mg/l and in two patients ceftriaxone only appeared in the sputum 12 h after the injection. All except one harboured beta-lactamase-producing Bran. catarrhalis in the sputum, and the possibility of breakdown of ceftriaxone by branhamella beta-lactamases is suggested.