A patient with malignant histiocytosis had an iron plasmatic concentration above 600 gamma/100 ml. An abnormal iron-binding protein incompletely saturated is demonstrated in the plasma. This protein has an electrophoretic migration near the alpha 2 globulins and no immunological reactions with antitransferrin and antiferritin antibodies. Its affinity for iron is less and its molecular weight is greater than the one of transferrin. Such a protein has not already been described in the plasma and its presence should be related to the malignant disease.