SUMMARY The erythrocyte glutathione reductase activity was measured in 199 healthy blood donors. The normal range was found to be 6&3 to 39 0 lU/109 erythrocytes. There was no variation related to either age or sex.The enzyme content of the red blood cell has been well reviewed by Prankerd (1961) and by Pennel (1964). It is now well established that variations in both the content and activity of the erythrocyte enzymes can occur in health (McDonald and Huismann, 1962;Bernard, Dubin, Madec, and le Cam, 1966; tSwierczewski !and Gibelin, 1965;Haywood and Starkweather, 1968) and in disease (Bonsignore, Fornaini, Segni, and Fantoni, 1960;Carson, Brewer, and Ickes, 1961; Lohr and Waller, 1961; Fornaini, Bianchini, Leoncini, and Fantoni, 1964;Loder and de Gruchy, 1965;Beutler, 1968;Grimes, 1969; Necheles, Maldonado, Barquet Chediak, and Allen, 1969).The present study of erythrocyte glutathione reductase activity was carried out to develop a reliable, rapid assay method and to use it to find the normal range expressed in units related to the red cell mass. Pennel (1964) In order to obtain a statistically significant number of results the enzyme activity was assayed in 199 healthy blood donors and the data were analysed for evidence of variation with age or sex (Table I).