A study conducted on Fe2+ autoxidation showed that its rate was extremely slow at acidic pH values and increased by increasing the pH; it was stimulated by Fe3+ addition but the stimulation did not present a maximum at a Fe2+/Fe3+ ratio approaching 1:1. The species generated during Fe(3+)-catalyzed Fe2+ autoxidation was able to oxidize deoxyribose; the increased Fe2+ oxidation observed at higher pHs was paralleled by increased deoxyribose degradation. The species generated during Fe(3+)-catalyzed Fe2+ autoxidation could not initiate lipid peroxidation in phosphatidylcholine liposomes from which lipid hydroperoxides (LOOH) had been removed by treatment with triphenylphosphine. Neither Fe2+ oxidation nor changes in the oxidation index of the liposomes due to lipid peroxidation were observed at pHs where the Fe3+ effect on Fe2+ autoxidation and on deoxyribose degradation was evident. In our experimental system, a Fe2+/Fe3+ ratio ranging from 1:3 to 2:1 was unable to initiate lipid peroxidation in LOOH-free phosphatidylcholine liposomes. By contrast Fe3+ stimulated the peroxidation of liposomes where increasing amounts of cumene hydroperoxide were incorporated. These results argue against the participation of Fe3+ in the initiation of LOOH-independent lipid peroxidation and suggest its possible involvement in LOOH-dependent lipid peroxidation.