To investigate the molecular mechanisms of the anti−apoptosis effects of hematopoietic growth factors GM−CSF/IL−3 fusion protein, GM−CSF and IL−3 in Tf−1 cells induced by γ−irradiation. Caspase−3 activity was detected using fluorescence chromatometry. RT−PCR semiquantitative analysis was used to identify the expression changes of Bcl−2 and Caspase−3 mRNA, and an immunohistochemical technique was used to examine the protein expression in irradiated Tf−1 cells. The Caspase−3 activity in irradiated Tf−1 cells was markedly reduced to 15.71%—43.65% in GM−CSF/IL−3 fusion protein, GM−CSF, IL−3, and the combination of GM−CSF and IL−3−treated groups compared to those in the control groups without any growth factor. Bcl−2 mRNA and its protein expression were significantly increased by the above cytokines after incubation with the cells for 48h after irradiation, while the enhancement of the Bcl−2 mRNA level by GM−CSF/IL−3 fusion protein (10μg/L) began from 24h. After 48h of incubation after irradiation, the expression of Caspase−3 mRNA was dramatically augmented by GM−CSF/IL−3 fusion protein at 10μg/L, 100μg/L and IL−3 10μg/L, as was the corresponding Caspase−3 protein expression by the addition of each cytokine. The cytokines exerted the anti−apoptosis effects in irradiated Tf−1 cells through the up−regulation of Bcl−2 and Caspase−3 expression, and the inhibition of Caspase−3 activity.