Extracellular-superoxide dismutase (EC-SOD) is one of the SOD isozymes mainly distributed in the extracellular fluid. In the vascular system, it is located on the endothelial cell surface according to studies on the heparin binding capacity. By measurement of serum EC-SOD levels of Japanese in healthy persons (n = 103) and hemodialysis patients (n = 150), 7 healthy subjects and 24 hemodialysis patients were classified into group II associated with high EC-SOD levels. By molecular analysis of the EC-SOD coding region from the group II individuals in Sweden, a single nucleotide substitution of G to C generating an amino acid change of arginine to glycine has been identified in the region associated with the heparin affinity of the enzyme. The same mutation was detected in the Japanese as a homozygote in both alleles of 2 hemodialysis patients and as a heterozygote in one allele of all the healthy group II individuals and 17 hemodialysis patients. The amino acid substitution may result in the decrease of the heparin affinity which is favorable for the existence of EC-SOD in the serum.