Provided are novel superoxide dismutase derivatives represented by the following general formula (I) <IMAGE> (I) wherein [SOD] is a superoxide dismutase residue derived by removal of two mercapto groups, and W is a divalent long chain hydrocarbon residue which may optionally be interrupted by one or more groups each independently selected from the group consisting of an oxygen atom, a sulfur atom and a group of -N(R)- (R being a lower alkyl group). The SOD derivatives retain most of the enzymatic activities of unmodified SOD and have much longer plasma half-life than unmodified SOD. They are effective for treating various diseases caused by active oxygen species. Also provided are chemical modifiers to prepare the above SOD derivatives.
