AbstractMurine marrow cells, when incubated in methylcellulose culture in the presence of erythropoietin and conditioned medium for two weeks, produced large macroscopic bursts containing normoblasts, macrophages, and often megakaryocytes. The clonal nature of these mixed colonies was supported by linearity studies and analyses of the percentages of constituent cells in different plating conditions. Time course observations and studies of the effects of L‐cell‐conditioned medium revealed that colony‐forming cells for the mixed colonies (CFU‐mix) are at earlier stages of hemopoietic development than burst‐forming units (BFU‐E). The mean of the modal sedimentation velocities of CFU‐mix was 3.4 mm/hr and was in close agreement with that reported for the spleen colony‐forming units. Almost none of the CFU‐mix was in a DNA synthetic phase as measured by short‐term exposure to tritiated thymidine. These results strongly indicate that CFU‐mix represent a population of pluripotent hemopoietic stem c