Extracts and fractions prepared from five medicinal herbs used to treat cancer in Chinese folk medicine were tested for their antiproliferative activities on five cancer cell lines using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. The MTT results revealed that the ethanolic extracts possessed stronger antiproliferative activities than the corresponding aqueous ones. The bioactive fractions of the ethanolic extracts were mostly found in n-hexane and/or ethyl acetate fractions, which by TLC analysis were shown to contain terpenes and/or phenolic compounds. The ethyl acetate fraction from the ethanolic extract of Elephantopus scaber showed the highest effect with IC50 values of less than 25 microg/mL on three susceptible cell lines, namely, PC-3 (human prostate carcinoma cell), CNE (human nasopharyngeal carcinoma epithelial cell) and HL-60 (human acute promyelocytic leukemia cell). Further isolation and analysis of this fraction led to the identification of three sesquiterpene lactones, two flavonoids and five triterpenes. MTT results showed that the sesquiterpene lactones were the most effective group with IC50 values ranging from 0.9 to 7.5 microM while the flavonoids possessed moderate to strong effect with IC50 values ranging from 17.3 to 42.6 microM. However, the triterpenes displayed weak effect with IC50 values larger than 50 microM. Among the active compounds, deoxyelephantopin, a sesquiterpene lactone, exhibited the strongest effect on the PC-3, CNE and HL-60 cells, with IC50 values of 4.6, 2.6 and 0.9 microM, respectively. Flow cytometric analysis showed that treatment with deoxyelephantopin caused subG-1 population augmentation in PC-3, CNE and HL-60 cells, suggesting apoptosis was induced in these cells.