Radionuclide studies of the erythron are valuable to the physician in evaluating the clinical situation in a wide variety of hematologic disorders. A complete and accurate analysis of the life cycle of the red cell can be obtained with a full iron kinetic study, in conjunction with a DF 32P red-cell survival study. However, a complete iron kientic study is not always necessary. It may be abbreviated by deleting the in vitro phase of the iron kinetic procedure. The abbreviated iron kinetic study is also done in conjunction with a DF 32P red-cell survival study. It can easily be performed by injecting 59Fe-labeled plasma and monitoring externally over the spleen, liver, and sacrum. Measurements of red-cell survival may be obtained with either 51Cr or DF 32P. Although 51Cr provides a relatively uniform label of circulating red cells and is convenient to count in vitro, its highly variable elution rate precludes an accurate measurement of erythrocyte survival. The 51Cr method provides only a rough index of circulating red-cell half-times as a measure of red-cell survival. DF 32P, however, is a permanent label of circulating red cells. It provides a direct measurement of erythrocyte survival and permits in vivo labeling of red cells simply by means of direct intravenous injection. Because it has an elution rate that is virtually zero after minimal elution on the day of injection, and because it is not reutilized, DF 32P is unquestionably the best agent known for the determination of red-cell survival. In addition to these diagnostic data, the complete iron kinetic study can provide data on the deposition of iron in storage and the rate of iron storage exchange. It can also determine if erythropoiesis is quantitatively abnormal and if the abnormality is located in the bone marrow or in other organs such as the liver or spleen. Although the study of hematologic disorders is one of the most rapidly developing areas of medical research, techniques that are currently available can provide an understanding of the life cycle of the red cell and valuable data that can be applied directly to the clinical situation. When performed accurately, these studies provide a thorough analysis of the pathophysiology of the erythron and are valuable clinical tools that can be used successfully in the diagnosis and evaluation of a broad spectrum of hematological disorders.