Aim To investigate radioprotective effects of platelet factor 4 (PF4) on hemopoiesis of mice after γ -ray irradiation and their mechanisms. Methods The mice were injected with PF4 twice at 6h intervals; and 20 h after the second injection they were given one irradiation of 7.5Gy 60Co γ -ray. 8d later the colony forming unit of granulocyte-macrophage (CFU-GM), colony forming of unit spleen (FU-S), number and DNA contents in bone marrow cells (BMCs) were examined by flow cytometry. Results 8d after irradiation, a significant difference was shown between DNA contents in BMCs and cell cycles of the PF4-injected mice and those of the irradiated mice (P∨ 0.01), whereas little difference was found between those of the irradiated mice and the controls (P∧ 0.05). Conclusion PF4 can obviously protect hemopoietic stem cells (HSCs) of mice from radiation injury and promote proliferation of HSCs after radiation.