High lipid peroxidation rates and lower antioxidative protection were found in the serum of 64 patients with gastric (n = 20) and duodenal (n = 44) ulcers complicated by hemorrhage. The administration of zantac, a histamine H2-receptor blocker, considerably reduced lipid peroxidation and enhanced antioxidative activity, which positively affected the clinical course of diseases. The findings have led to the conclusion that zantac may have antioxidative properties.