Objective：To investigate the effects of estradiol on 60Co γ-ray induced apoptosis of bone marrow hematopoietic cells of mice,and to discuss the related anti-irradiation mechanism.Methods: KM mice were randomly divided into 3 groups (15 mice/each group): control group (without radiation),pure radiation group and estradiol + radiation group (ER group).The pure radiation group was irradiated by 4.0 Gy γ-ray at a dose rate of 1.15Gy/min; the ER group was administered with 0.1 mg estradiol (IM) at 10 days before 4.0 Gy γ-ray radiation; and the control group received no special treatment.The apoptotic DNA segments of bone marrow hematopoietic cells were analyzed by DNA agarose gel electrophoresis; flow cytometry was used to examine the apoptosis rate of cells and expression of Fas and Bcl-2 at 4 h,8 h,and 12 h after irradiation.Results: Eight hours after radiation,the apoptotic DNA segments were obviously increased and apoptotic DNA ladder appeared,which was not seen in the other 2 groups.The apoptosis rate of bone marrow hematopoietic cells in ER group was significantly lower than that in the pure radiation group at 4,8, and 12 h after irradiation(P<0.01).Fas expression was only slightly elevated in the ER group than in the control group, but was markedly lower than that in pure radiation group(P<0.01),and showed no typical crest-time； Bcl-2 expression was significantly higher in the ER group than in the pure radiation group(P<0.01).Conclusion: Estradiol can decrease the apoptosis of mice bone marrow hematopoietic cells induced by γ-ray,probably through down-regulation of Fas and up-regulation of Bcl-2 expression.