Doxycycline levels were measured fluorimetrically in the serum and pulmonary tissue of 36 patients undergoing a pulmonary operation. Doxycycline 200 mg was given intravenously 20 hours before operation. The doxycycline concentration in pulmonary tissue (1.03 to 12.30 mug/g) were as high or higher than the serum doxycycline levels (0.94 to 7.82 mug/ml) in the majority of cases. This is possible as the lung has an ample blood supply and doxycycline is a lipophilic drug. There were large variations in the results, which was probably related to the underlying illness and to an altered blood circulation. The side effects were minimal. The daily infusions of doxycycline were continued over 5 days, no conclusions could be drawn about its possible prophylactic effect on postoperative infections.