The therapeutic use of cell-free haemoglobin as a blood substitute has been hampered by toxicological effects. A model asolectin (phosphatidylcholine/phosphatidylethanolamine) liposome system was utilized to study the pro-oxidant efficiency of several chemically modified haemoglobins on biological membranes. Lipid peroxidation, resulting from the interactions between haemoglobin and liposomes, was measured by conjugated diene formation and the maximal rates of oxygen uptake. Spectral changes gave insight into the occurrence of the ferryl iron species. The residual reactivity of oxidatively damaged haemoglobins with ligands during incubation with liposomes was assessed from rapid kinetic carbon monoxide-binding experiments. Liposomes in which cytochrome c oxidase was embedded show both haemoglobin and the enzyme to be oxidatively damaged during incubation. The functional state of cytochrome c oxidase was monitored in the presence and absence of a free radical scavenger. Once in contact, both unmodified and modified haemoglobins triggered and maintained severe radical-mediated membrane damage. Differences in the pro-oxidant activities among haemoglobins may be explained by either the differential population of their ferryl intermediates or disparate dimerization and transfer of haem into the membrane with subsequent haem degradation. This study may contribute to a better understanding of the molecular determinants of haemoglobin interactions with a variety of biological membranes.