1. An in vitro method to observe radiosensitivity of stem cells was developed in the present study. In vivo and in vitro effect of 60Co irradiation on the erythropoietin-induced stem cell differentiation into erythroblasts was observed, using a tissue culture method of polycythemic mouse spleen. Response to erythropoietin was demonstrated by an appearance of heme synthesis and erythroblasts in spleen fragments.

2. A significant correlation between the rate of appearance of erythroblasts and heme synthesis of the spleen fragments was observed.

3. After irradiation, marked impairment of both heme synthesis and production of erythroblasts was observed, yielding D37 values in the vicinity of 70 r in vivo and 120 r in vitro irradiation, respectively.

4. Marked recovery of erythropoietin-induced heme synthesis in the polycythemic mouse spleen was observed 9 days after 300 r irradiation, with an "overshooting" phenomenon on the 12th day.

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