1. The activity of monoamine oxidase, when assayed with four substrates, was significantly lowered in platelets prepared from the blood of patients with iron-deficiency anaemia. 2. Treatment with oral iron preparations restored platelet monoamine oxidase activity to normal in those patients whose serum iron concentrations also returned to normal. 3. Platelet monoamine oxidase activity remained low if treatment failed to restore serum iron concentration to within normal limits.