To improve the in vivo pharmacological potential of Cu, Zn-superoxide dismutase (Cu,Zn-SOD), human recombinant Cu,Zn-SOD was conjugated with succinylated keratin fragment (Suc-ker). Suc-ker-conjugated Cu,Zn-SOD (Suc-ker-SOD) was formed by attachment of about 3.7 mol of Suc-ker to 1 mol of Cu,Zn-SOD and exhibited an apparent mean molecular weight of 107 kDa. Suc-ker-SOD exhibited 74.1% SOD activity on a molar basis. When Suc-ker-SOD was administered i.v. into mice, its plasma half-life was prolonged to 20.5 min compared with 4.7 min for native SOD. After i.v. administration of 51Cr-labeled proteins into mice, native SOD was excreted rapidly into urine and no significant accumulation was observed in organs other than the kidneys. On the other had, Suc-ker-SOD was taken up rapidly by the liver. Furthermore, Suc-ker-SOD exhibited much lower immunogenicity and much better therapeutic effects than native and poly(styrene-co-maleic acid)-conjugated SOD against paraquat toxicity in mice. Moreover, the inactivation rate of Suc-ker-SOD by H2O2 was less than those of native SOD and polyethylene glycol-conjugated SOD. The effect of H2O2 on liganded Cu++ in the active site of Suc-ker-SOD was much less than those of other SOD derivatives. These results indicate that Suc-ker-SOD has advantages over the other SOD derivatives tested.