20 patients who underwent thoracotomy and lung operations mainly for lung cancer, were given oral doxycycline (DC) 100-4-- mg the previous evening. Blood samples were taken for the fluorometric assay of DC before the anaesthetic, during the operation, and 2-3 hours postoperatively. Serum DC levels during the operation correlated approximately with the dose of DC, and they were parallel to the DC concentrations measured in pulmonary or/and bronchial tissue, 11 out of 20 patients showed elevated serum DC levels during recovery, non-specific fluorescence being excluded. This phenomenon was ot seen in experiments with rabbits in which the decline of the blood DC levels was assessed after the intravenous injection of DC. Possible causes for elevated serum DC are discussed.