The distribution of phospholipids over the outer and inner layers of the plasma membranes of differentiated Friend erythroleukemic cells (Friend cells) and mouse reticulocytes has been determined. Phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol were found to be distributed symmetrically over both layers, sphingomyelin was found to be enriched in the outer layer (80-85%) and phosphatidylserine appeared to be present mainly in the inner layer (80-90%) of the plasma membranes of differentiated Friend cells. The outer layer of reticulocyte membranes contains 50-60% of the phosphatidylcholine, 20% of the phosphatidylethanolamine, 82-85% of the sphingomyelin and 40-42% of the phosphatidylinositol. All of the phosphatidylserine is present in the inner layer. The results show, that the asymmetric distribution of phospholipids, typical for erythrocyte membranes, is partially apparent already at an early stage of erythropoiesis, the proerythroblast, while the final organization of phospholipid distribution takes place at some stage during enucleation of the enormoblast and release of the reticulocyte into the blood stream.