The effects of low-dose preirradiation on the process of radiation-induced cell death were investigated in human leukemic MOLT-4 cells. By 0.2 Gy of X-rays given 12 h prior to a challenge dose of 5 Gy, the process of apoptosis was accelerated. The acceleration was associated with a certain increase in caspase 3 activity, a disruption of the mitochondrial transmembrane potential, and an accumulation of p53 proteins. This finding is in contrast to the radiation adaptive responses in which a small dose of preirradiation would induce certain radiation resistance and decrease the cell death after irradiation with higher doses.