Ferritin was purified from serum of patients with idiopathic haemochromatosis. Analysis on SDS electrophoresis showed that it is composed of two major bands of 19,000 and 23,000 Mr. The smaller peptide has an electrophoretic mobility and immunochemical reactivity similar to that of tissue L subunit. The larger, previously named G subunit, is recognized by concanavalin-A and by anti ferritin L-subunit, but not by anti-H, monoclonal antibodies. All of the antibodies show higher affinity for the L than for the G subunit. Therefore, the G chain appears immunochemically similar, but not identical, to ferritin L chain, and is responsible for serum ferritin binding concanavalin-A.