A kindred with hereditary spherocytosis was found to have erythrocyte glutathione reductase deficiency. The glutathione reductase was evaluated enzymatically and immunologically. The enzyme activity could not be restored either by flavin adenine dinucleotide (FAD) in vitro, or by oral administration of FAD. Kinetic and electrophoretic studies, heat stability tests, and pH curves revealed no evidence of a variant enzyme. The results of electroimmunodiffusion and neutralization tests using an antiserum indicated that the deficiency is caused by a diminished production of enzyme protein. Hereditary spherocytosis and glutathione reductase deficiency traits segregated independently. The amount of reduced glutathione in the erythrocyte was normal, and the deficiency state without hereditary spherocytosis was asymptomatic. All deficient members in this kindred were heterozygous and the residual enzyme activity in the heterozygote might be sufficient to maintain the reduced glutathione level to prevent oxidative stress.