1. 1. A further characterization and localization of the membrane receptor for transferrin on rat reticulocytes is described. PAGE studies with a purified membrane complex B 2, from which the functional role in transferrin binding and iron uptake has been shown previously, showed that the transferrin receptor is localized on a membrane protein with a mol. wt of ~ 70–80.10 3. 2. 2. Selective solubilization of the rat reticulocyte membrane has shown that this receptor protein belongs to one of the minor integral membrane polypeptides, embedded in the lipid bilayer of the membrane. 3. 3. Proteolipid complexes, glycolipids and sialoglycoproteins of the rat reticulocyte membrane play no direct role in the binding capacity of the receptor.