2',4'-Dihydroxy-6'-methoxy-3',5'-dimethylchalcone (DMC), isolated from the buds of Cleistocalyx operculatus, was investigated in its cytotoxicity and its influence on six human cancer cell lines. Among SMMC-7721, 8898, HeLa, SPC-A-1, 95-D and GBC-SD cell lines, SMMC-7721 cells was the most sensitive one in these tested cell lines, with IC50 equal to 32.3 +/- 1.13 microM, EC50 equal to 9.00 +/- 0.36 microM and the therapeutic index equal to 3.59. Staining with Hoechst 33258 showed fragmentation and condensation of chromatin in the cells treated with 9 microM DMC for 48 h. Flow cytometric analysis was performed to determine hypodiploid cells. The results of flow cytometry assay indicated that the percentage of hypodiploid SMMC-7721 cells were 49.44 +/- 1.06% after 48 h treatment with 18.0 microM DMC. The treatment resulted in the appearance of a hypodiploid peak (A0 region), probably due to the presence of apoptosing cells and/or apoptotic bodies with DNA content less than 2n. To our knowledge, this is the first report on anti-tumor activity by DMC.