The diffusion of Enoxacin into the bronchial mucus was studied in 34 patients admitted because of an acute infectious episode over chronic bronchopathy. They received via the oral route 400 mg of Enoxacin twice daily. The antibiotic concentrations were measured in sputum and serum by microbiological assay and HPLC. The patients were divided into 5 groups according with the timing of sampling: 1, 3, 6, 12 hours after dosing at day 3 (group I, II, III, IV), 3 hours after dosing at day 1, 2 and 3 (group V). Serum concentrations of Enoxacin were low (2.46 mg/l at 3rd hour). Bronchial concentrations exceed blood levels (mean 3.06 mg/l at 3rd hour). The bronchial levels of Enoxacin were above the mean MIC of many respiratory bacterial pathogens.