EARLIER work1,2 has indicated a disturbance in iron metabolism in avian erythroblastosis, a virus disease of chickens in which there is an erythroblastic hyperplasia of the bone marrow with the entry of primitive red cells into the circulating blood. In normal birds injected intravenously with iron-59, the radioistope disappears from the plasma and reappears in 24–48 h. In birds with erythroblastosis the activity reappears more quickly1, autoradiographic investigations in blood smears suggesting that most of the activity is associated with the primitive cells2.