The sera of patients with idiopathic haemochromatosis and iron-overload have been found to contain low-molecular-weight iron complexes detectable in the 'bleomycin assay'. These complexes stimulate both the peroxidation of membrane lipids and the formation of the highly reactive and damaging hydroxyl radical. The iron chelator desferrioxamine interferes with these reactions. We suggest that oxygen radical reactions stimulated by iron salts are important in the pathology of idiopathic haemochromatosis.