Previous studies have shown that erythroid precursors at sequential stages of differentiation along the red cell pathway can be distinguished by differences in the size and maturation kinetics of the colonies to which they give rise in vitro. Using criteria based on these two parameters, it is thus possible to identify three distinct erythroid progenitor cell populations in the mouse, known as day 8 BFU- E, day 3 BFU-E, and CFU-E. These cell types have now been shown to differ in a number of other respects, including progenitor cell size, sensitivity to cycle-active agents, response to plethora, and effects of the W/Wv genotype. In addition, a comparison of the differences found between day 8 BFU-E and day 3 BFU-E on one hand and those distinguishing day 3 BFU-E and CFU-E on the other provides support for the view that early erythropoietic cell differentiation involves a series of changes that take place long before competence to synthesize hemoglobin becomes manifest.