Human hemoglobin has been used as a pro-oxidant for artificial unilamellar phospholipid vesicles, containing cytochrome-c oxidase inserted into the bilayer. This experimental system was suitable to follow directly the kinetics of lipid oxidation and the effects on both the vesicle membrane permeability and the functional state of cytochrome-c oxidase. Following mixing of vesicles with hemoglobin, an oxygen dependent, peroxyl radical mediated, rapid oxidation (taking a few minutes) of the lipid was found to occur. On a similar time scale the membrane became ion-leaky and cytochrome-c oxidase damaged. The pro-oxidant effects of hemoglobin in various oxidation and ligation states were studied and a mechanism, based on a ferric/ferryl redox cycle of the heme-iron is proposed to account for these observations. © 1994.