The relationship between haemoglobin and membrane oxidation was studied using liposomes containing haemoglobin (haemosomes) as a red cell model. Rapid oxidation occurred in haemosomes formed from purified haemoglobin and unsaturated lipid (egg phosphatidylcholines). After 3 h at 37 degrees C most of the haemoglobin was oxidized, predominantly to methaemoglobin with some haemichrome formation. The oxidation of haemoglobin was paralleled by membrane lipid peroxidation as measured by thiobarbituric acid reactivity. These changes were largely abolished by using freshly prepared haemolysate instead of purified haemoglobin, or when haemosomes were prepared with saturated phosphatidylcholines. In haemosomes consisting of fresh haemolysate and saturated phosphatidylcholine, the rate of haemoglobin oxidation at 37 degrees C corresponded to that of non-encapsulated haemolysate, and after 4 months storage at 4 degrees C 45% of oxyhaemoglobin was oxidized. In haemosomes prepared from purified haemoglobin and egg lecithin, alpha-tocopherol, catalase and ascorbate each protected against both haemoglobin oxidation and lipid peroxidation. Superoxide dismutase or reduced glutathione had no effect. In unsaturated-lipid haemosomes containing haemolysate, the rate of haemoglobin oxidation increased when catalase was inhibited or reduced glutathione was depleted, but after long term incubation only concurrent catalase-inhibition and glutathione depletion could increase thiobarbituric acid reactivity. These results demonstrate a close interdependence between haemoglobin oxidation and lipid peroxidation, and show that constituents of haemolysate strongly protect against both processes. H2O2 appears to be an important mediator, with its removal by either catalase or the glutathione/glutathione peroxidase system protecting against both oxidative changes.