Ferritin, the major iron storage protein in mammalian cells, was treated with various concentrations of different oxidants: xanthine/xanthine oxidase, Sin‐1 (3‐morpholinosydnonimine, purchased from Alexis, Grunberg, Germany), DEA‐NO (Diethylamine NONOate, purchased from Calblochem‐Novabiochem, Schwalbach, Germany), and hydrogen peroxide. The proteolytic susceptibility towards the isolated 20S proteasome of untreated ferritin and oxidized ferritin was measured in parallel with the iron liberated by these oxidants. With increasing hydrogen peroxide, Sin‐1, and xanthine oxidase concentrations, the measured proteasomal degradation of ferritin also increased. At higher oxidant concentrations, however, the proteolytic susceptibility began to decrease. The oxidation of ferritin by DEA‐NO was accompanied by a lesser increase of proteolytic susceptibility in comparison with the effects of the other oxidants. Addition of DEA‐NO to Sin‐1 suppressed the increase in proteolytic susceptibility of ferritin, whereas adding xanthine/xanthine oxidase had no additional effect. Iron was liberated readily from ferritin as a result of the oxidation process, although the increase in proteolytic susceptibility was not always correlated to the iron release. In fact, the degradation of oxidatively damaged ferritin was not accompanied by a further increase of free iron. Therefore, we conclude that the proteasome is a secondary antioxidative defense system that degrades only nonfunctional ferritin.