The study was conducted by using leukocytes isolated from 74 apparently healthy donors and 60 patients with bronchial asthma. The generation of active oxygen forms was determined by luminolo- and lucigenin-dependent chemiluminescence techniques and NTC-reaction. The findings suggest that at the tension close to the blood tension of 37.5 mm Hg and the high tension of 146 mm Hg is a powerful natural inhibitor of leukocytic generation of active oxygen forms. At an exacerbation, the inhibitory effect of carbon dioxide on the leukocytic generation of active oxygen forms decreased in most (70%) patients with bronchial asthma, which potentiates the free radical mechanism of development of bronchial asthma. It may be held that the literature-described use of carbon dioxide for the treatment of bronchial asthma is justifiable only in a lower proportion of patients who have preserved a high sensitivity to the inhibitory effect of carbon dioxide on the generation of active oxygen forms.