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To investigate effects of ( Z )-3-chloromethylene-5-methyl-thiochroman-4-ketone (CMTK) on apoptosis in human breast cancer MCF-7 cells in vitro , cultured MCF-7 cells were treated with different concentrations of CMTK. The MTT method was used to study inhibition rates; the trypan blue method for cell viability and to construct growth curves; flow cytometry for cell cycles; the TUNEL assay and flow cytometry to assess proportion of apoptotic cells; Ac-IETD-pNA caspase-8/Ac-LEHD-pNA caspase 9 Colorimetric Assay Kit for caspase-8 and caspase-9 activities; and ELISA assays to detect DR3, TNFR1, TNF-α and caspase-3 proteins. Results showed that CMTK had dramatic anti-tumor activities at low concentrations (IC 50 : 14.24 ± 0.12 μmol L -1 ). Importantly, the IC 50 of CMTK on MCF-7 cells was much higher than that of cisplatin. Growth curves show CMTK can inhibit cell proliferation in time- and dose-dependent manners. In cell-cycle analysis, proportions of MCF-7 cells in G 0 /G 1 phase after 12-h and 24-h treatment with CMTK were significantly different from the control group. In contrast, the proportion of cells in S phase tended to decrease, but not significantly. Use of TUNEL and flow cytometry indicated that CMTK induces apoptosis. In MCF-7 cells, CMTK induces TNFR1 production, activates caspase-8 and caspase-9, and enhances caspase-3 levels in a concentration-dependent way; however, CMTK does not affect production of DR3 and TNF-α. These results suggest that the anti-tumor mechanism of CMTK involves inducing apoptosis via caspase-8 activation by death receptor, and caspase-9 activation by the mitochondrial-dependent pathway. Our results provide preliminary data for further investigation of the apoptotic mechanism.