On 12 patients with bronchial carcinoma serum and lung-tissue concentration levels of tetracyclin were measured. The concentration levels of the antibiotic were twice as high in the lung tissue as in the serum. In the lower lobe of the lung significantly higher values were measured than in the upper lobe. The demonstrated tissue levels of Pyrrolidino-methyltetracyclin (Reverin) in human lung are high enough to be effective in pulmonary bacterial infection.