The concentrations of cefpirome, a new semi-synthetic cephalosporin, in the bronchial mucosa and serum were assessed after a single 1 g intravenous dose in 37 patients. Bronchoalveolar lavage allowed epithelial lining fluid (ELF) concentrations to be measured in eight subjects. The mean concentration for serum was 34.5 mg/L (S.E.M. 3.3), for bronchial mucosa 19.3 mg/kg (S.E.M. 1.9) and for ELF 7.2 mg/L (S.E.M. 1.1). The progressive reduction in cefpirome concentration in serum compared to that in bronchial mucosa and ELF is consistent with the permeability characteristics of beta-lactam antimicrobials and the barriers to movement of antimicrobial agents present in the lung.