The protective effect of 0.166 mg estradiol benzoate intramuscularly 10 days prior to total body irradiation has been redemonstrated. One milligram estradiol in propylene glycol was as effective as the benzoate, but estrone and estriol were less so and long-acting estradiol compounds were ineffective. Radioprotection conferred by pretreatment with estradiol was associated with stimulation of hematopoietic colonies in the spleen and had no relationship to immunological competence as measured by hemolytic plaque-forming cells to sheep erythrocytes. Since estradiol appeared to reduce the number of stem cells capable of migrating to the spleen and forming new hematopoietic colonies, the beneficial effects appear to be due to daughter cells that arise during the 10 days before irradiation. Whether their apparent resistance is simply due to increased numbers, or to a qualitative change in sensitivity related to cell cycle remains to be answered.

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