Reticulocytes were separated on the basis of density by isopycnic centrifugation in dextran gradients. This parameter was shown to correlate with the degree of maturity of the cells. Lactoperoxidase iodination of cells of different densities followed by sodium dodecyl sulfate (NaDodSO4) electrophoresis revealed a 190 000 molecular weight protein which was well labeled in early reticulocyte membranes. Efficiency of labeling decreased as the cells increased in density, and high specific activity iodination of mature erythrocytes did not result in the labeling of any species near this molecular weight. Inclusion of rabbit transferrin prior to the iodination procedure resulted in a specific loss of labeling of this 190 000 molecular weight species. When steps were taken to clear endogenous transferrin from the membranes, the labeling of this species was enhanced. These observations are consistent with the concept that transferrin can block the lactoperoxidase catalyzed iodination of this membrane protein by specifically associating with it. Coomassie blue and periodic acid-Schiff staining of NaDodSO4 gels of these membranes revealed that a glycoprotein present at this molecular weight is lost during the course of reticulocyte maturation. It is concluded from these studies that a glycoprotein of molecular weight 190 000 constitutes the transferrin receptor in the reticulocyte membrane.