Ozone autohemotherapy is used in the treatment of some diseases. Carbonic anhydrases (CAs, EC 18.104.22.168) are metalloenzymes and play a role in homeostatic mechanisms. The aim of this study was to investigate the effects of ozone on human red blood cell CA (hCA) enzyme activity. Blood samples were treated with different doses of ozone (10, 20, 30 mu g/ml) and the erythrocyte total CA activities were determined. Also, purified hCAI and hCAII isozymes were treated with the same doses of ozone and the enzyme activities were measured. About 30 mu g/ml ozone treatment decreased the purified hCAI and hCAII activity and increased the total CA activity compared to the control. Because the implication of CAs on many physiological and biochemical processes is linked to pathologies, it can be suggested that the ozone at a concentration of 30 mu g/ml is safely used by autohaemotherapy in a well-designed clinical trial.