Pefloxacin is a new quinolone with significant in vitro antibacterial activity and a broad spectrum. It was included in an extensive study of antibiotic diffusion into bronchial mucus. Pefloxacin was given to 35 patients divided into five groups according to the timing of sputum sampling. Plasma specimens were taken simultaneously. Pefloxacin was measured by microbiological assay. Mean pefloxacin concentrations in bronchial mucus was high, exceeding 5 mg/l. Concentrations on the first and third days were similar. The ratio of mucus concentrations to plasma concentrations was high in each case, around or often above 1. These findings show that diffusion of pefloxacin into bronchial mucus is outstanding.