To overcome several disadvantages of the use of superoxide dismutase(SOD)in its native form the surface modification of therapeutically useful enzyme by covalent linkage of nontoxic,nonimmunogenic, and biocompatible polymers has been investigated.Recombinent human Cu/Zn SOD (rh Cu/Zn SOD) obtained from E.coli was coupled chemically with activated polyethylene glycol(PEG).The modified rhCu/Zn SOD,with apparent molecular weight about 60000 daltons,has longer half-life(15 hours), and the remaining enzyme activity is more than 80 per cent. Its stabilities against acid, alkali and heat were increased significantly. The relation between the amount of PEG used and the remained enzyme activity was also reported.