Ferrous citrate Fe 59 is indicated, by intravenous administration, to determine various parameters of the kinetics of iron metabolism, including plasma iron clearance, plasma iron turnover rate, and the utilization of iron in new red blood cells. The values of serum iron obtained from these studies provide diagnostic information in patients with anemias. Ferrous citrate Fe 59 is also useful to assess the role of the spleen in red blood cell production and destruction, and thus to help determine the advisability of splenectomy. Also, organ uptake measurements are used to measure the sites of red cell production (or lack thereof) in extramedullary erythropoiesis in myeloproliferative disorders. Iron from ferrous citrate is bound to plasma protein (transferrin) and carried to the blood-forming organs where it is utilized to form hemoglobin or is deposited in the reticuloendothelial cells of the liver and spleen. The amount of radioactive iron absorbed, transported, stored, utilized, and excreted can then be measured by the periodic collection of blood specimens and external counting.