Apoptosis is a common mode of cell death after exposure of tumor cells to radiation and/or chemotherapy. The factors that determine the rate of induction of apoptosis are generally related to the functioning of cell cycle checkpoints. In the present study, we investigated the involvement of several genes in cell cycle redistribution and induction of apoptosis in U937 cells after low and high doses of radiation. Activation of CDC2 was observed after both low and high doses of radiation in U937 cells that underwent apoptosis. Expression of CDK2, CDC2 and cyclin A was induced rapidly in the process of radiation-induced apoptosis. In addition, we investigated the use of a clinically relevant dose of radiation to promote As2O3-induced apoptosis in U937 cells. We found that combining radiation and As2O3 may be a new and more effective means of cancer treatment.