30 bronchial asthma (BA) patients and 15 donors were exposed to 8.3% and 20.8% CO2 to bring out leukocytes sensitivity to CO2 by generation of active oxygen (AO) in bronchial asthma. CO2 effects on leukocyte AO generation were defined by luminol-dependent chemiluminescence (CL) before and after the exposure to CO2. It was found that in healthy subjects 8.3% and 20.8% CO2 noticeably inhibits leukocyte CL. However, in 70% of asthmatics with BA exacerbation leukocyte sensitivity to CO2 inhibition diminished. This was normal in 30% of BA patients. With BA aggravation, leukocyte sensitivity to CO2 tended to a decrease. Remission brought a complete or partial recovery of the above sensitivity. Thus, on the one hand, CO2 is involved in BA pathogenesis in terms of its inhibitory effect on AO generation by leukocytes; on the other hand, only in 30% of BA patients high CO2 concentrations as a treatment may be justified.