Penetration of ofloxacin into bronchial secretions following a single 400 mg oral dose was studied in 52 patients. Bronchial secretions were aspirated at bronchoscopy between one and 24 h after dosing and blood was collected at the same time. Mean bronchial secretion concentrations between 2 and 2.6 mg/l were measured in 36 patients between 1 and 6 h but there was considerable individual variation in both serum and bronchial aspirate concentrations. Concentrations declined slowly between 9 and 24 h to a mean value of 0.4 mg/l at 24 h. Correlation between serum and sputum concentrations of ofloxacin was fairly good despite marked deviation in some individuals. The ratios of the concentration in bronchial secretion to serum were remarkably constant at 1 to 6 h, varying between 0.96 during the third and fourth hours and 0.81 in the sixth hour. The results suggest that ofloxacin in a 400 mg oral dose is likely to achieve therapeutic activity against respiratory pathogens including Streptococcus pneumoniae, with sustained activity against more sensitive organisms for up to 24 h.