Primitive and definitive erythropoiesis each share 3 phases of differentiation. Committed erythroid progenitors give rise to maturing nucleated erythroblasts that ultimately generate enucleated erythrocytes. The specific erythroid progenitors are EryP-CFCs (primitive erythroid colony-forming cells), BFU-Es (erythroid burst-forming units), and CFU-Es (erythroid colony-forming units). The characteristics common to primitive and definitive erythroblast maturation in mammals are listed along with references pertaining to primitive erythropoiesis in the mouse. We propose that a major distinction between primitive and definitive erythropoiesis in mammals is that primitive red cells differentiate while circulating in the bloodstream, whereas definitive red cells enter the circulation only after completing their maturation extravascularly.