Vesicles are released during the in vitro culture of sheep reticulocytes which can be harvested by centrifugation at 100,000 X g for 90 min. These vesicles contain a number of activities, characteristic of the reticulocyte plasma membrane, which are known to diminish or disappear upon reticulocyte maturation. The activities include acetylcholinesterase, cytochalasin B binding (glucose transporter) nucleoside binding (i.e. nucleoside transporter), Na+-independent amino acid transport, and the transferrin receptor. Enzymes of cytosolic origin are not detectable or are present at low activity in the vesicles. Cultures of whole blood, mature red cells, or white cells do not yield comparable levels of these activities, supporting the conclusion that the activities arise from the reticulocytes. In addition, the lipid composition of the vesicles shows the high sphingomyelin content characteristic of sheep red cell plasma membranes, but not white cell or platelet membranes, also consistent with the conclusion that the vesicles are of reticulocyte origin. It is suggested that vesicle externalization may be a mechanism for shedding of specific membrane functions which are known to diminish during maturation of reticulocytes to erythrocytes.