To investigate the possible mechanism of ${alpha}-spinasterol$ as a candidate of anti-cancer drug, I examined the effects of ${alpha}-spinasterol$ on the apoptosis of U937 cells MTT assay, flow cytometric analysis, SDS-polyacrylamide gel electrophoresis, Western blot analysis, and RT-PCR were performed. ${alpha}-spinasterol$ treatment reduced the cell viablilty of U937 cells in a dose-dependent manner, which was associated with the induction of apoptotic cell death. ${alpha}-spinasterol$ treatment also reduced the levels of Bcl-xL anti-apoptotic protein expression and increased the levels of caspase-3, p53, pro-apoptotic protein, in U937 cells. After treatment the level of Bcl-xL, anti-apoptotic gene expression was decreased and the level of ICE pro-apoptotic gene expression was increased. These findings suggest that ${alpha}-spinasterol$ induced the apoptotic cell death via regulation of several growth regulatory gene products. The abbreviations used are: FBS, fetal bovine serum; PBS, phosphate buffered saline; PI, propidium iodide; OD, optical density; DiOC6, 3,3-dihexyloxa carbcyanine iodide; MTT, 3 [4-5-dimethylthiazol-2-yl] -2-diphenyltetrazolium bromide.