In order to elucidate the effects of hemoglobin on the oxidative damage, the oxidations of soybean phosphatidylcholine (PC) liposomes and low density lipoprotein (LDL) in aqueous dispersions induced by ruptured erythrocytes (hemolysate) and methemoglobin were studied. The rate of oxidation of soybean PC liposomes induced by hemolysate was much faster than that induced by the same concentration of ferric sulfate or cupric chloride and increased with the increase in the amount of hemolysate. When soybean PC was treated beforehand with ebselen, which decomposes hydroperoxides but can not scavenge free radicals, the oxidation did not proceed. Furthermore, the oxidation was suppressed by the addition of pentamethylchromanol in liposomal membrane. The reaction of hemoglobin with methyl linoleate hydroperoxide in aqueous dispersions was also studied. It was found that hemolysate and methemoglobin decomposed the hydroperoxides much faster than cupric chloride and ferric sulfate. The electron spin resonance study strongly suggested that both hemolysate and methemoglobin decomposed hydroperoxides rapidly to form alkoxyl and peroxyl radicals. It was also found out by spectrophotometrical study and by using carbon monoxide and potassium cyanide that hemolysate was oxidized by organic hydroperoxide to form methemoglobin and ferrylhemoglobin. The rate of oxidation of LDL induced by hemolysate was much slower than that of soybean PC liposomes, while LDL was oxidized faster than soybean PC liposomes by cupric chloride under the same conditions. However, in the presence of tert-butyl hydroperoxide, the oxidation of LDL induced by hemolysate proceeded rapidly. These results were interpreted by both the chemical reactivity and the accessibility of metal catalyst to the lipid hydroperoxides in LDL and liposomes. It was suggested that hemoglobin released by hemorrhage might play a crucial role in the initiation of oxidative damage in vivo.