During the maturation of the mammalian reticulocyte to the erythrocyte, 50nm bodies (called exosomes) are released from the cell. These exosomes carry the transferrin receptor (TFR) as well as other plasma membrane and lysosomal activities which the cells are known to lose during maturation. Using an antibody attached to an iron core bead, we have shown that other plasma membrane activities (nucleoside transporter and acetylcholine esterase) are packaged in the same exosome as the transferrin receptor. This population of exosomes is devoid of lysosomal enzymes. Exosomes were detected in the circulation of several species of phlebotomized animals, suggesting that exosome formation is a natural phenomenon, and probably a route by which the red cell decreases in size and sheds specific plasma membrane activities during maturation.