The mean activity of glutathione peroxidase (GSH-PX) in erythrocytes of 22 Israeli-Jewish patients with multiple sclerosis (19.3 ± 4.5 U/gHb) was significantly lower than in a control group of 30 Jewish patients with various neurological disorders (24.3 ± 5.1 U/gHb). This observation confirms a similar finding of a decreased activity of GSH-Px in erythrocytes of multiple sclerosis patients in Denmark (Shukla et al. 1977). These results are discussed in relation to the possibility of selenium deficiency and to the recently described genetic polymorphism and ethnic variation of GSH-Px activity in human red cells. It is concluded that additional investigations are required in order to elucidate the cause of the decreased activity of this enzyme in red cells of patients with multiple sclerosis.