The activities of 4 enzymes, i.e. alkaline phosphatase,γ-glutamyl transferase, lactate dehydrogenase and creatine kinase were studied in bronchial aspirates and serums from two groups of subjects, the first one was composed of 14 subjects without active bronchopulmonary pathology and the other of 20 patients with lung cancer. The results showed a statistically significant decrease of the activities of alkaline phosphatase and ß-glutamyl transferase in bronchial aspirate from patients with bronchogenic malignant tumors in relation to normal subjets. This finding could be explained by the ‘fetalism’ principle, which states that the quantitative pattern of enzymes of immature human tissues resembles those of neoplastic tissues.