Among multiagent mechanisms causing peptic ulcer disease, oxygen free radicals hypothesis is reasonable. Our investigation concerned the determination of oxygen free radicals generation lipid peroxidation and enzymatic antioxidative defense in blood of patients with peptic ulcer disease. The examinations were performed in 79 males, non-smokers, aged 20-51 with peptic ulcer disease, clinically diagnosed as gastric or duodenal ulcer. Biochemical investigations were carried out on blood platelets obtained from venous blood according to the centrifugation fractionation technique. Superoxide dismutase (CuZn-SOD) (EC 188.8.131.52.) activity and malonyl dialdehyde (MDA) concentration were determined according to Misra and Fridovich (1972) and Placer et al (1966) methods, respectively. Oxygen free radicals as the superoxide anion O2- generation in blood according to Bellavite et al. (1983) method was determined. The results indicate that patients with peptic ulcer disease have a higher level of malonyl dialdehyde in blood platelets, whereas the superoxide dismutase activity is significantly decreased in comparison with the control group. Oxygen free radicals generation both in patients with peptic ulcer disease as in healthy men is maintained at normal level. Our data suggest that in spite of similar range of oxygen free radicals generation both in patients with peptic ulcer disease and healthy men, the injury to patients by superoxide anion is more destructive and harmful to cells, because the enzymatic antioxidative defense is decreased. The increase of lipid peroxidation in patients with peptic ulcer disease also supports this conclusion.