The invention provides a superoxide dismutase derivative of the general formula[SOD][Z]wherein [SOD] represents a superoxide dismutase having 1 to 22 or 24 groups each derived from an amino groups by removal of one hydrogen atom in lieu of amino groups; [Z] represents a monovalent copolymer group, constituting units of which are a group of the formula ##STR1## wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 each represents a hydrogen atom or a residue derived by a removal of a hydroxyl group from an alkanol of 1 to 8 carbon atoms, an ethylene glycol monoalkyl ether containing an alkyl moiety of 1 to 4 carbon atoms or a glycerin dialkyl ether containing alkyl moieties of 1 to 4 carbon atoms, provided that either R.sup.1 or R.sup.2 and either R.sup.3 or R.sup.4 each represents a hydrogen atom, and a residue derived from the group of the above-mentioned formula by removal of OR.sup.1, OR.sup.2, OR.sup.3 or OR.sup.4 group from one of its COOR.sup.1, COOR.sup.2, COOR.sup.3 and COOR.sup.4 groups (where the bond on the carbon atom of the carbonyl group is attached to [SOD]), said monovalent copolymer group having an average molecular weight of 500 to 200,000; and n represents an integer of 1 to 22 or 24 corresponding to the number of said groups each derived from an amino group by removal of one hydrogen atom in said [SOD], or a pharmaceutically acceptable salt thereof. There also is provided a method for producing the above derivative and salt. The invention is further directed to medicinal uses and pharmaceutical compositions for oral administration.