Objective:To study the effects of the Thevetoside(TS),which is one of the cardiac glycosides,as an inhibitor of Na+-K+-ATPase,on K562 cells cultured in vitro and its antitumor mechanism,Objective:The cytotoxicity of TS on K562 cells was determined by trypan blue dye exclusion,The inhibitory effect of Na+-K+-ATPase of TS on K562 cells by 86Rb tracing method.The morphological changes of TS on K562 cells were observed under transmission electronic microscope.Results:The studies on antitumor activities in vitro showed that TS at low dose(0.001～0.1μg/ml),with dose dependence,was able to kill K569 cells significantly.But the drug-induced cell lethality was displayed and strengthened quickly·only when the cells were continuously treated by TS more than 8 h.The growth of K562 cells was notabaly inhibited after they were exposed to 0.1μg/ml、TS for l2 h.TS remarkably inhibited the Na+-K+-ATPase activity of K562 cells at 0.001～0.1μg/ml for l h.Under electronic microscope,the K562 cells exposed to TS for 24 h displayed morphological changes of swelling and necrosis.Conclusion:TS showed potent cytotoxicity on K562 cells.Its mechanism was probably attributable to inhibit the Na+-K+-ATPase activity of K562 cell membrane,resulting in metabolic perturbation of water and ions,and disturbing absorption of nutrients.