A comparative, preliminary study of the phagocytic capacity of polymorphonuclear leukocytes, and of the total plasma antioxidants was carried out in a group of workers professionally exposed to mineral dusts and in a group of patients with lung cancer. Both groups exhibited advanced respiratory failure stages II and III. The phagocytic capacity of polymorphonuclear leukocytes was found significantly increased in workers with pneumoconioses and moderately increased in patients with lung cancer. On the other hand, the peroxides and total plasma antioxidants exhibited great individual variations. Thus in lung cancer it was observed that peroxides had a tendency to decrease and antioxidants to increase. The individual study of the variations of these three parameters might be able to differentiate the reversible from the irreversible stages of the respiratory changes.