From rat bone marrow in which erythropoiesis has been depressed (by suppression of erythropoietin production), liquid culture has yielded a predominantly synchronous amplification of erythroblast development. Groups of 2, 4, 8, 16 and 32 erythrolbasts are observed at 9-15 hour intervals in successive stages of maturation, similar (by their staining characteristics and nuclear size) to that seen in vivo.