A membrane protein with specific transferrin binding activity has been isolated from rabbit reticulocytes. The isolation procedure involved the immunoprecipitation by antibody to transferrin of transferrin-receptor complexes from reticulocyte membrane proteins which had been solubilized with nonionic detergent. Receptor dissociated from the antibody-transferrin-receptor complexes could bind transferrin saturably and reversibly. It migrated electrophoretically as a single band of glycoprotein with an estimated MW of approximately 180,000 which was reduced to around 93,000 following complete dissociation with dithiothreitol.