Human hepatoma cells hepG2 were irradiated twice by ,60,Co γ-rays with a priming dose of 5cGy and a higher dose of 3Gy performed 4h or 8h after the low dose irradiation. Effects of the priming dose irradiation on cell cycle arrest caused by high dose were examined with flow cytometry. Cells in G,2,/M phase accumulated temporarily after the 5cGyirradiation, and proliferation of tumor cells was promoted significantly by the low dose irradiation. After the 3Gy irradiation, G,2, phase arrest occurred, and S phase delayed temporally. In comparison with 3kGy irradiation only, the priming dose delivered 4h prior to the high dose irradiation facilitated accumulation of hepG2 cells in G,2,/M phase, whereas the priming dose delivered 8h prior to the high dose irradiation helped the cells to overcome G,2, arrest. It was concluded that effects of the priming dose treatment on cell cycle arrest caused by high dose irradiation were dependent on time interval between the two irradiations.