The effects of betulinic acid (BA),a pentacyclic lupane-type triterpene,on the cell viability,cell cycle and apoptosis in human leukemia K562 cells were investigated.The effects of BA on the growth of K562 cells were studied by MTT assay.Apoptosis was assayed through Annexin V/propidium iodide (PI) double-labeled cytometry.The effects of BA on the cell cycle of K562 cells were studied by a PI method.The expression of Bax and capase-3 was detected by using Western blot.The results showed that BA was cytotoxic to K562 cells with an IC50 of 21.26 μg/mL at 24 h.After treating K562 cells with 10 μg/mL BA for 72 h,the number of cells was reduced by 58%.BA induced apoptosis of K562 cells in a time-and dose-dependent manner.The proportion of cells in G0/G1 and G2/M phases was decreased and that in S phase was increased after K562 cells were treated with BA for 24 h.BA treatment also increased the expression of the pro-apoptotic proteins Bax and caspase-3.It suggested that BA could inhibit the proliferation of K562 cells through the induction of cell cycle arrest and apoptosis.The antitumor effects of BA were related with up-regulation of the expression of Bax and caspase-3 proteins.BA may qualify for the development of new therapies for leukemia.