BACKGROUND Haematoxylum brasiletto is a tree that grows in Central America, commonly known as "Palo de Brasil," which is used in the traditional medicine for the treatment of cancer and gastric ulcers. OBJECTIVE The aim of this study was to isolate the compounds responsible for antiproliferative activity of H. brasiletto. MATERIALS AND METHODS A bioassay-guided fractionation of ethanol extract of H. brasiletto was performed using 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide cell proliferation assay to measure the antiproliferative activity on six human cancer cell lines (A549, LS180, HeLa, SiHa, MDA-MB-231, and NCI-H1299) and one human noncancer cell line (ARPE-19). The ethanol extract was partitioned with hexane, dichloromethane, and ethyl acetate. The active dichloromethane fraction was fractioned by silica-column chromatography, and active subfractions were separated using preparative-thin layer chromatography. The chemical structure of an isolated compound was elucidated with different chemical and spectroscopic methods. RESULTS The flavonoid brazilin (1) was isolated from the heartwood of H. brasiletto. The measurement of antiproliferative activity showed that brazilin can inhibit the growth of SiHa, MDA-MB-231, A549, and NCI-H1299 cell lines by 50% at doses of 44.3, 48.7, 45.4, and 48.7 μM, respectively. Furthermore, the flavonoid showed a high antiproliferative activity on LS 180 and HeLa with IC50 values of 62.2 and 71.9 μM, respectively. Brazilin also exhibited a high antiproliferative activity on the human noncancer cell line ARPE-19 with an IC50 value of 37.9 μM. CONCLUSIONS Brazilin: (6aS, 11bR)-7,11b-Dihidro-6H-indeno[2,1-c] cromeno-3,6a, 9,10-tetrol was isolated; this compound demonstrated antiproliferative activity against several human cancer cell lines. This work demonstrated that brazilin, a flavonoid isolated and characterized of H. brasiletto, has antiproliferative activity against cancer cell lines. SUMMARY The flavonoid brazilin was isolated from the heartwood of H. brasilettoBrazilin is able to inhibit the growth of SiHa, MDA-MB-231, A549 and NCI- H1299 cancerous cell linesBrazilin exhibited a moderate antiproliferative activity on the human non-cancer cell line ARPE-19Brazilin demonstrated to have antiproliferative activity against human cancer cell lines and could be a potential source of anticancer agents. Abbreviations used: MTT: [3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium]; FBS: Fetal bovine serum; TLC: Thin layer chromatography.