Apart from a knowledge of pharmacokinetic characteristics of an antibacterial agent, the measurement of the amount of active drug at the site of infection provides important information when monitoring antibiotic distribution. Antibiotic concentrations in bronchial secretions have been evaluated in various studies in animals (1) and in humans (2); these studies represent the closest available approximation to the site of broncho-pulmonary infections. This is of particular value in cystic fibrosis (CF) which is a heritable disease characterized above all by a chronic obstructive pulmonary pathology and also by digestive and urinary-tract disorders (3). Chronic pulmonary infections which are the hallmark of cystic fibrosis require repeated courses of treatment with potent antibiotics. Most pulmonary exacerbations of cystic fibrosis are due to Pseudomonas aeruginosa (4, 5). Antibiotics suited for this particular situation must be active against this pathogen.