After a single oral 600 mg dose, ofloxacin concentrations were measured in lung tissue, whole blood and plasma in 11 patients undergoing thoracotomy for a bronchial malignancy. To correct for blood contamination in the tissue samples, the tissue haemoglobin content was measured using a method based on the binding of haemoglobin by haptoglobin. Ofloxacin concentrations in plasma and whole blood did not differ significantly. The calculated blood content in the tissue samples was 0.12 +/- 0.05 ml/g lung tissue. After correction for blood admixture, the mean lung tissue concentration 2 h after administration of ofloxacin was 17.7 +/- 9.2 micrograms/g. At the same time the mean plasma concentration was 8.7 +/- 4.2 mg/l (P less than 0.02). The high concentration of ofloxacin obtained in lung tissue does not result from the preparation technique. After a single 600 mg dose the tissue concentrations proved to exceed MIC values for most pathogens frequently involved in respiratory tract infections.