Exposure of superoxide dismutase to hydrogen peroxide or ascorbate -Fe(III) gives rise to the modification of the enzyme structure and the changes of its function as well as of its immunological property. The structure alterations are especially shown as the loss of the enzyme-bound copper, the production of peptide fragments, the formation of carbonyl groups in the amino acid side chains, the increase in the contents of aspartic acid, glutamic acid and glycine and a decrease in the number of histidine, proline, arginine, lysine, serine and threonine. Accompanying the structure changes, the enzyme lost its catalytic activity. Hydrogen peroxide and ascorbate -Fe(III) exhibited different effects on the structural and functional changes of the enzyme. Furthermore, the bovine superoxide dismutase exposed to hydrogen peroxide or ascorbate -Fe(III) with different kinds of antisera such as antibovine, anticanine, antiporcine and antihuman superoxide dismutase antisera all showed greatly increases in the reactivities. It is proposed that reactive oxygen species may play a role in the building up of circulating immune complexes in autoimmune diseases.