The amount of stainable iron in sternal aspirates has been estimated by the use of the prussian blue reaction. The superiority of histologic sections over smears for the purpose of iron staining has been stressed. The results of applying this simple histochemical test routinely to sections prepared from 141 marrow samples are described. It is shown that in disorders of the blood the procedure provides a simple means for the control of iron therapy. Not only does it indicate when iron is required but also when enough has been given and so can be used to obviate the danger of overdosage whets iron is being administered by the intravenous route. The method is put forward as a possible means of distinguishing the hypochromic anemias of chronic intoxication from those due to iron deficiency.