Iron transport in plasma is carried out by transferrin, which donates iron to cells through its interaction with a specific membrane receptor. The soluble form of the transferrin receptor has been identified in plasma. Dosage of the soluble receptor represents a simple, reliable, quantitative assay of erythropoiesis, which should replace the standard ferrokinetic measurement. Using this dosage, it is now possible to investigate the pathophysiology of anemia in many conditions, and to analyze the erythropoietic response to treatment with recombinant erythropoietin.