It is clear that cytokines regulate normal hematopoiesis including proliferation, differentiation, and apoptosis (programmed cell death). Leukemia is a disorder of unbalance between excessive proliferation and inappropriate apoptosis. Leukemic cells are also stimulated by cytokines similar to normal hematopoietic cells. It is reported that the oncogenes and tumor suppressor genes are associated with not only leukemogenesis but also cell cycle, for example proliferation, differentiation, and apoptosis (programmed cell death). We found that G-CSF leads to apoptosis of radiation induced murine leukemia cell line (C2M-A5), and its apoptosis is dependent on cell cycle. From these results together with other related reports, cytokines seem to be a key substance of apoptosis of leukemic cells and the apoptosis inducing therapy will be a new strategy for leukemia therapy.