An improved plasma culture system is described for the production of erythrocytic colonies by mammalian adult hemopoietic cells in vitro under the influence of erythropoietin. The concentration of fetal calf serum in the medium used for dilution of the cells was critical for erythrocytic colony formation when low numbers of cells were plated. Optimal concentrations were found for plasma, fetal calf serum, bovine serum albumin, and L-asparagine in the culture medium, and the colony-forming efficiency was shown to depend on the concentration of erythropoietin. With erythropoietin at plateau concentration, the number of erythrocytic colonies produced was directly proportional to the number of bone marrow or spleen cells plated, over a wide range of cell concentrations. Colony numbers per culture conformed to a Poisson distribution. Thus, the improved plasma culture system may be used for the quantitative assay of CFU-E. The method is rapid (2 days), reliable, convenient, and inexpensive. Since the improved plasma culture system also supports granulocytic colony formation by bone marrow cells in the presence of conditioned medium (CSA), and the number of granulocytic colonies produced is proportional to the number of cells plated, the same hemopoietic cell suspensions can be simultaneously assayed for CFU-E and CFU-C under virtually identical conditions.