A technique for observing in flask cultures the differentiation and proliferation of murine CFU-S into each of the three myeloid lines is described. The differentiation of cells along the erythroid pathway is arrested at a very early stage making it possible to obtain large numbers of primitive erythropoietic cells derived from adult murine bone marrow. The pattern of differentiation obtained from these BFU-E suggests that the capacity for granulopoiesis is usually lost before a committment to either megakaryopoiesis or erythropoiesis is made.