We examined the effect of low-dose γ-irradiation on proliferation of myeloid leukemia, Ml cells. Ml cells irradiated with 4 cGy showed increased proliferation as determined by cell number, alamar Blue metabolism and DNA synthesis. The increase by low-dose irradiation was observed at every concentration of serum from 1 % to 10%. showing the decrease of the serum requirement in irradiated cells. Furthermore, survival days of SL mice inoculated intraperitoneally with low-dose irradiated Ml cells were shorter than those with sham-irradiated cells. These results shown as the decrease of serum requirement and increase of transplantation ability, indicated the augmentation of leukemia malignancy by low-dose irradiation.