The aim of this study was to investigate the radioprotective effect of recombinant murine interleukin 12 (rmIL-12) on mice irradiated by γ-ray. Fifty- six BALB/c mice were totally irradiated by 6.0 Gy of (60)Co γ-ray and randomly divided into irradiation control group,rmIL-12 treated group and recombinant murine thrombopoietin (rmTPO) treated group.The 5 and 20 µg/kg of rmIL-12 were administrated intraperitoneally at 24 h before irradiation respectively (low and high dose rmIL-12 treated group), 15 µg/kg of rmTPO was administrated subcutaneously at 30 min and 24 h following irradiation in rmTPO treated group. The general conditions of mice were observed twice a day, the changes in body weight and peripheral blood cell counts were examined once every three days, bone marrow cells were collected to perform colony cultivation at day 14 and 28 after irradiation. The results showed that the general conditions of mice in rmIL-12 treated group were better than those in irradiation control group. Compared with the irradiation control group,5 and 20 µg/kg rmIL-12 treatment significantly promoted platelet recovery, resulting in less profound nadirs (15.9% vs 8.1%,18.2% vs 8.1%,P < 0.01) and rapid recovery to normal levels (11 days vs 14 days). WBC count recovery rate in rmIL-12 treated group was faster than that in the irradiation control group. The WBC and platelet count recovery rate in 5 µg/kg rmIL-12 treated group were as fast as that in the rmTPO treated group, both of which were slower than that in 20 µg/kg rmIL-12 treated group (P > 0.05). Semi-solid bone marrow cell culture also demonstrated that rmIL-12 could stimulate bone marrow cells to form more CFU-Mix than those in the irradiation control group in vitro at day 14 and 28 after irradiation(P < 0.01).There was no significant difference between rmIL-12 and rmTPO treated groups (P > 0.05), CFU-GM counts in 5 µg/kg rmIL-12 treated group and rmTPO treated group at day 28 after irradiation were higher than those in irradiation control group(P < 0.05), but less than those in 20 µg/kg rmIL-12 treated group (P < 0.05). It is concluded that rmIL-12 has a significant radioprotective effect on mice irradiated by γ-ray.