Carbon dioxide (CO2) influence in generation of active oxygen forms (AOF) in human mononuclear cells (blood phagocytes and alveolar macrophages) and animal cells (tissue phagocytes, parenchymal and interstitial cells of liver, kidney, lung, brain, and stomach) was investigated. The AOF generation was examined by the methods of chemiluminescence (CL) using luminol, lucigenin, and NBT (nitroblue tetrazolium) reaction. It was established that CO2 in concentrations similar to those in the blood (5.1%, pCO2 37.5 mmHg) and at high concentrations (8.2%, pCO2 60 mmHg; 20%, pCO2 146 mmHg) showed a pronounced inhibitory effect on the AOF generation in all the studied cells (usually reducing it 2 to 4 times). Those results were obtained not only after the direct contact of isolated cells with CO2 but also after the whole body’s exposure to CO2. Besides, it was established that the venous blood gas mixture (CO2 – 45 mmHg, +O2 – 39 mmHg, + N2 – 646 mmHg) inhibited the AOF generation in cited cells more than the arterial blood gas mixture (CO2 – 40 mmHg, + O2 – 95 mmHg, + N2 – 595 mmHg). The carbon dioxide action mechanism was developed partially through the inhibition of the OAF generation in mitochondria and through the deceleration of NADPH oxidative activity. Finally, it was established that CO2 led to better coordination of oxidation and phosphorylation and increased the phosphorylation velocity in liver mitochondria. The results clearly confirmed the general property of CO2 to inhibit significantly the AOF generation in all the cell types. This favors the new explanation of the well-known evolutionary paradox: the Earth life and organism’s preservation when the oxygen, which shows toxic effects on the cells through the AOF, occurs in the atmosphere. The results can also be used to explain in a new way the vasodilating effect of CO2 and the favorable hypercapnotherapy influence on the course of some bronchial asthma forms. The results are probably significant for the analysis of important bio-ecological problem, such as the increase of CO2 concentration in the atmosphere and its effect on the humans and animals.