Therapeutic strategies for leukemia are directed to induction of differentiation and apoptosis as well as growth inhibition. One of the key antileukemic agents, 1-β-D-arabinofuranosylcytosine (ara C), is clinically applied according to these therapeutic aims. However, the molecular effects of 0.1 μg/ml of ara C, a concentration that corresponds to the serum level in leukemic patients on a conventional dose of ara C, have not been well disclosed. Here, we addressed these issues using K562 cells which derived from a blastic crisis of chronic myeloid leukemia. DNA synthesis of treated cells was suppressed from 1–6 h. But, it recovered at 12 h and no further inhibition was observed. The number of cells was not decreased but DNA fragmentation was observed at 72 h. The number of erythroid-differentiated cells also increased to 30% at 72 h. Along with treatment, no marked alteration of mRNAs for cell cycle-regulating genes was found and the retinoblastoma gene product remained hyperphosphorylated throughout treatment. The expression of mRNAs for apoptosis-regulating genes also remained unchanged, except for slight down-regulation of Bax. c-myc protein was not found later than 48 h, and Max mRNA was downregulated. c-jun was immediately induced, followed by the fluctuated expression level along with treatment. These findings suggest that the 0.1 μg/ml ara C changed the proliferation, differentiation and death of K562 cells in a biphasic manner. In the early phase, DNA synthesis was inhibited without altering the expression of cell cycle regulating-genes. In the latter phase, cell death and erythroid- differentiation occurred in accordance with the down-regulation of c-myc.