We constructed conjugates of superoxide dismutase (SOD) and the Fc fragment of human immunoglobulin G. The lysyl residues of bovine erythrocyte Cu,Zn-SOD were covalently linked with cysteine residues of the Fc fragment using N-succinimidyl 4-(N-maleimido)-butylate as a crosslinking agent. Analysis by gel filtration and SDS-PAGE revealed that the conjugates were composed of one molecule of SOD linked with one molecule of Fc [SOD-(Fc)1] and one SOD molecule linked with several Fc molecule [SOD-(Fc)n]. The resulting SOD-Fc conjugates retained more than 90% of the enzyme activity of SOD. When those conjugates were administered intravenously to mice, the half-lives of SOD activity in the circulation were 29 and 42 h for SOD-(Fc)1 and SOD-(Fc)n, respectively, while free SOD had a half-life of 5 min. Intravenous administration of the conjugates to mice markedly repressed the increase in serum glutamic-oxaloacetic transaminase (GOT) activity induced by paraquat. These results suggest that SOD-Fc conjugates, which have long half-lives, effectively perform dismutation of superoxide radicals and may be useful for preventing tissue injury caused by hazardous oxygen metabolites.