It is shown that an in vitro system of assaying the size of an erythropoietin-responsive stem cell pool could be applied to the spleens of polycythemic mice after irradiation and bone marrow transplantation.

With this method, the presence of erythropoietin-responsive cells in the spleen was first detected on the second day after transplantation. Therefore, it is considered probable that colony-forming cells and erythropoietin-responsive cells are at different stages of maturation or cell cycle.

Furthermore, necessity of erythropoietin for further differentiation of transplanted stem cells into erythroblasts is also suggested.

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