To investigate a possible link between subretinal hemorrhage and macular degeneration, oxyhemoglobin (HbO2) or methemoglobin (metHb) was incubated with retinal homogenate and unsaturated phospholipid peroxidation was monitored by (a) assay of thiobarbituric acid-reactive substances (TBARS), (b) luminescence originating from an energy transfer of lipid-degraded products to rose bengal, and (c) the decrease in composition of highly unsaturated fatty acids of phospholipids. TBARS formation and rose bengal luminescence in the case of metHb-induced lipid peroxidation were about 1.5 times greater than those in HbO2-induced lipid peroxidation. alpha-Tocopherol, a lipid-soluble antioxidant, and docosahexaenoic acid, a major unsaturated fatty acid, were slightly more rapidly decomposed after a 60-min incubation with metHb than with HbO2 at the same concentration. Atomic absorption analysis revealed that an equal concentration of iron was released from both HbO2 and metHb during incubation with retinal homogenates. The released iron may promote microsomal phospholipid peroxidation in the presence of endogenous ascorbate or NADPH-dependent cytochrome P-450 reductase because ascorbate oxidase and p-chloromercuribenzoic acid (an inhibitor of sulfhydryl enzymes) inhibited metHb- or HbO2-induced lipid peroxidation. MetHb-induced lipid peroxidation in retina was inhibited by KCN or NaN3, which binds to FeIII of metHb. KCN or NaN3 had no effect on HbO2-induced lipid peroxidation, because conversion of HbO2 to metHb, which can proceed in HbO2 incubated with phospholipid liposome, did not occur in retinal homogenates. It is concluded that metHb induces peroxidation of retinal unsaturated phospholipids (1) directly and (2) by releasing iron.