Natural dicarbonyls, which may be accumulated during oxidative stress in atherosclerosis (e.g. malondialdehyde) or carbonyl stress in diabetes mellitus (glyoxal and methylglyoxal) effectively inhibited the activities of commercial preparations of antioxidant enzymes: catalase, Cu, Zn-superoxide dismutase (Cu, Zn-SOD) and Se-contained glutathione peroxidase from human and bovine erythrocytes and also rat liver glutathione-S-transferase. After incubation of human erythrocytes with 10 mM of each investigated dicarbonyls the decrease of intracellular Cu, Zn-SOD was observed. The decreased activity of erythrocyte Cu, Zn-SOD was also detected in diabetic patients with carbohydrate metabolism disturbance but effective sugar-lowered therapy was accompanied by the increase of this enzyme activity. The increase of erythrocytes activity of Cu, Zn-SOD of diabetic patients theated with metformin (which may utilize methylglyoxal) was higher than in erythrocytase of diabetic patients subjected to traditional therapy.