In Down's syndrome the extra chromosome in no 21 yields increased activity of superoxide dismutase (SOD) in the erythrocytes. In a study of 15 patients with Down's syndrome 6 patients were treated with sodium selenite. Untreated patients presented significant lower concentration of zinc and increased concentration of copper and titanium in the erythrocytes, increased copper concentration in the neutrophil granulocytes and mononuclear monocytes. Treating with sodium selenite resulted in an increased number of selenium bearing mononuclear cells and decreased copper concentration but the copper concentration in erythrocytes and neutrophil granulocytes was not altered. S-Cu, S-Zn, S-Se, S-Ca was not altered in the two groups. The activity of SOD and GSH-Px in the erythrocytes was increased in comparison with controls.