OBJECTIVE To explore the protection function of rapamycin in hematopoietic system damage induced by irradiation. METHODS Six to eight week old C57BL/6J male mice were used for experiment. Mice received 4 mg/kg rapamycin by i.p.injection every other day for 5 times. The day after the last injection, mice were exposed to a dose (5 Gy) of total body irradiation (TBI). Peripheral blood was measured by a complete blood count at 0.5, 1, 2, 3, 5, 7, 40, 70 days after TBI. The hematoxylin-eosin staining was used to observe the pathologic changes in sternum obtained from mice at day 5 after TBI. CFU-S of spleen was measured by immerging in Tellyesniczky solution for 24 h at day 5 after TBI. RESULTS Before TBI, WBC and LYM decreased in rapamycin-treated mice compared with control (P<0.01); RBC and HGB increased (P<0.05); there was no difference in PLT; HE staining of bone marrow from rapamcin-treated and control mice before irradiation showed no difference in marrow cellularity. After TBI, WBC and LYM decreased significantly, with no difference at 0.5 d to 7 d between rapamycin-treated and control. The counts of WBC and LYM in rapamycin-treated mice restored to normal at 40 d and 70 d. RBC and HGB decreased at irradiation group at 3 d to 7 d, but rapamycin stimulated them to a higher level, both of them tended to normal at 40 d and 70 d. HE staining of bone marrow after 5 day of 5 Gy irradiation, nucleated cells in control decreased significantly, but restored in rapamycin-treated mice. CFU-S results showed the colony number in rapamycin-treated mice was much higher than control mice after 5 Gy irradiation, with 40.00±12.86 and 13.20±2.31 (P=0.035), respectively. CONCLUSIONS Administration of rapamycin to mice before irradiation protected the mice from hematopoietic damage induced by irradiation by maintaining the bone marrow nucleated cells, slowing down decrease and promoting the restoration of peripheral blood cells and protecting hematopoitic stem/progenitor cells in spleen.