Purified erythroid progenitor cells (CFU-E) were used to study in vitro the production of the proteins present in the plasma membrane and the membrane skeleton. At different stages of erythropoiesis incorporation of [35S]methionine was measured and membranes were isolated. Whereas incorporation in the total protein mass of the cells increased during erythropoiesis, the labeling of the membrane protein fraction decreased. The major erythrocyte membrane proteins were synthesized already in the CFU-E and continued to be made till the orthochromatic erythroblast stage. Band 3 protein, however, was made at a much lower rate. The incorporation in the late stages was only 5% of that in the CFU-E. The major changes in the protein composition of the membrane and its adherent skeleton occurred at the enucleation step.