Iron salts stimulate lipid peroxidation by decomposing lipid peroxides to produce alkoxyl and peroxyl radicals which initiate further oxidation. In aqueous solution ferrous salts produce OH. radicals, a reactive species able to abstract hydrogen atoms from unsaturated fatty acids, and so can initiate lipid peroxidation. When iron salts are added to lipids, containing variable amounts of lipid peroxide, the former reaction is favoured and OH. radicals contribute little to the observed rate of peroxidation. When iron is complexed with EDTA, however, lipid peroxide decomposition is prevented, but the complex reacts with hydrogen peroxide to form OH. radicals which are seen to initiate lipid peroxidation. Superoxide radicals appear to play an important part in reducing the iron complex.