The presence of iron on the transferrin molecule increases its affinity for and sojourn time on the reticulocyte. This could be due to selective internalization of iron-containing transferrin molecules. This possibility was investigated by electron microscopic autoradiography. Rabbit reticulocytes were incubated with rabbit transferrin at 6, 33, and 72% iron saturations, and the distribution of transferrin molecules at membrane and intra-cellular locations was assessed by grain counting. The results showed that (1) both apo-transferrin and iron transferrin enter the cell interior and (2) the amount of intracellular transferrin was primarily controlled by the concentration of membrane-bound transferrin and not by its iron saturation.