1. Monoamine oxidase activity in platelets prepared from the blood of patients with iron-deficiency anaemia was significantly lowered when compared with that in platelets from normal subjects. 2. The Km values of the platelet enzyme for the substrates dopamine, 5-hydroxytryptamine, phenylethylamine and kynuramine were similar for the platelet enzyme from iron-deficient and normal groups. 3. Heat-inactivation studies showed that the platelet monoamine oxidase from iron-deficient subjects was more labile to this treatment, when compared with the platelet enzyme from normal subjects. 4. The sensitivity of platelet monoamine oxidase to the inhibitors, clorgyline and deprenil, was increased in iron-deficiency anaemia. 5. Binding studies with the 14C-binding irreversible monoamine oxidase inhibitor, deprenil, showed that the amount of enzyme capable of binding this inhibitor was lowered by 48% in platelets from iron-deficient patients when compared with platelets from normal subjects. 6. The results show that there is a lowered amount of active enzyme in platelets from iron-deficient subjects. It is suggested that iron is necessary either for the synthesis of monoamine oxidase apoenzyme or is a cofactor for an enzyme which attaches flavin—adenine dinucleotide covalently to the monoamine oxidase apoenzyme.