The ferritin concentration of the formed elements of the blood has been measured. Nearly equal amounts were found in plasma, red cells, granulocytes, and in the mononuclear (monocyte and lymphocyte) cell fraction. Platelets contained a negligible amount. Concentrations of ferritin were greatest in blood leukocytes, amounting to approximately 24 mug/ml. Changes occurring with iron depletion and iron overload were consistent with those expected in body tissues in general. With infection, increased levels of ferritin were observed in the mononuclear cell fraction, consistent with that which occurs in RE cells.