Aim:The aim of the present study was to investigate the inhibitory effect ofpseudolaric acid B (PAB) on human breast cancer MCF-7 cells.Methods:3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide analysis,morphologicalchanges,acridine orange staining,and agarose gel electrophoresis were appliedto detect apoptosis.The percentage of apoptotic and necrotic cells was calcu-lated by the lactate dehydrogenase activity-based cytotoxicity assay;senescenceassociated (SA)-β-galactosidase activity was detected to evaluate senescence;flow cytometric analysis of propidium iodide staining was carried out to investi-gate the distribution of cell cycle,and the protein expression was examined byWestern blot analysis.Results:During apoptosis,the half maximal inhibitoryconcentration IC_(50) was 3.4 and 1.35 μmol/L at 36 and 48 h after PAB treatment,respectively.The MCF-7 cells exposed to PAB showed typical characteristics ofapoptosis,including the morphological changes and DNA fragmentation.TheMCF-7 cells treated with 4 μmol/L PAB for 36 h underwent apoptosis,but notnecrosis.The apoptosis induced by PAB was independent of the death receptorpathway.The senescent cells became larger and flatter,and the SA-β-galactosi-dase staining was positive.PAB induced obvious mitotic arrest and it precededapoptosis and senescence.The expressions of p21 and p53 was upregulated withPAB treatment,and cyclin B 1 was upregulated and transported from the cyto-plasm to nuclei,and sustained stable levels.Conclusion:PAB induced mitoticarrest in the MCF-7 cells and inhibited proliferation through apoptosis andsenescence.The apoptosis was independent of the death receptor pathway. Aim: The aim of the present study was to investigate the inhibitory effect of pseudolaric acid B (PAB) on human breast cancer MCF-7 cells. Methods: 3- (4,5-dimethylthiazol-2-yl) -2,5-diphenyltetrazolium bromide analysis, morphologicalchanges, acridine orange staining, and agarose gel electrophoresis were appliedto detect apoptosis. The percentage of apoptotic and necrotic cells was calcu-lated by the lactate dehydrogenase activity-based cytotoxicity assay; senescenceassociated (SA) -β-galactosidase activity was detected to evaluate senescence; flow cytometric analysis of propidium iodide staining was carried out to investi-gate the distribution of cell cycle, and the protein expression was examined by Western blot analysis. Results: During apoptosis, the half maximal inhibitory concentration profile IC 50 1.35 μmol / L at 36 and 48 h after PAB treatment, respectively. MCF-7 cells exposed to PAB showed typical characteristics ofapoptosis, including the morphological changes and DNA fragmentation.Th eMCF-7 cells treated with 4 μmol / L PAB for 36 h underwent apoptosis, but not necrosis. The apoptosis induced by PAB was independent of the death receptorpathway. The senescent cells were larger and flatter, and the SA-β-galactosi-dase staining was positive. PAB induced obvious mitotic arrest and it has precededapoptosis and senescence. The expressions of p21 and p53 were upregulated with PAB treatment, and cyclin B 1 was upregulated and transported from the cyto-plasm to nuclei, and sustained stable levels. Conlusion: PAB induced mitoticarrest in the MCF-7 cells and laws proliferation through apoptosis and senescence. apoptosis is independent of the death receptor pathway.