Erythroblast enucleation is thought to be largely dependent on signals mediated by other cells, such as macrophages. In an attempt to improve the in vitro production of red blood cells (RBCs) from immature hematopoietic progenitor cells, we have developed a method to produce enucleated RBCs efficiently in the absence of feeder cells. Our method may represent an efficient way to produce transfusable RBCs on a large scale from hematopoietic progenitors.