The results testify to the fact that CO2 is a powerful inhibitor of reactive oxygen species (ROS) generation by cells (blood phagocytes and alveolar macrophages of 96 people and cells of inner organs and tissue phagocytes (of the liver, brain, myocardium, lungs, kidneys, stomach, and skeleton muscles), as well as by mitochondria of the liver of 186 white mice and human tissues. Generation of ROS was determined using various methods with CO2 directly acting on the cells and biopsies and indirectly on the organism as a whole. CO2 in the concentration of 5.1 % (P = 37.5 mmHg), 8.2 % (P = 60.0 mmHg), and 20 % (P = 146.0 mmHg) in a mixture with air (total pressure = 730 mmHg) inhibits the basal ROS generation by phagocytes on the average by 3.52, 5.69, and 10.03 times, respectively (p < 0.05), and the stimulated by corpuscular particles: (a) zymosan by 3.24, 4.43, and 7.95 times; (b)SiO2: by 2.99, 3.24, and 5.76 times (p < 0.05). This is confirmed by the fact that CO2, along with inhibiting the O2 − generation by cells of the various organs, including the liver, as a rule, by 2.19–4.7 times, p < 0.01 or <0.001 induces simultaneously a decrease in the O2 − generation by mitochondria isolated from the liver (by 1.91–3.2 times, p < 0.001). The mechanism of CO2 influence is realized, in part, by inhibition of NADPH-oxidase activity. Taking into consideration the proven role of CO2 in different pathophysiological conditions, (such as endoarteritis, bronchial asthma, and infectious diseases), present findings may be of clinical interest in terms of potential implementation of CO2 donors as adjuvant therapeutics in these diseases.