Bidens pilosa has been used as a folk medicine in various medications and as a popular ingredient in herb teas. Chemopreventive activities of crude and fractionated plant extracts of Bidens pilosa were evaluated in this study. Ethyl acetate and butanolic fractions, partitioned from the total crude extract of Bidens pilosa, exhibited significant scavenging free radical activity (IC(50) values approximately = with 14-17 microg/mL) comparable to that of alpha-tocopherol. Strong effects on the inhibition of LPS-mediated nitric oxide production in RAW 264.7 cells were also observed for the EA and BuOH fractions. Detectable cytotoxicity on RAW 264.7 cells, however, was observed for the EA fraction at a dose >100 microg/ml. The metabolite profile and major constituents of the BuOH fraction were studied and characterized using various spectroscopic analyses. A new compound, heptanyl 2-O-beta-xylofuranosyl-(1-->6)-beta-glucopyranoside (1), and eight phenolic compounds, namely quercetin 3-O-rabinobioside (2), quercetin 3-O-rutinoside (3), chlorogenic acid (4), 3,4-di-O-caffeoylquinic acid (5), 3,5-di-O-caffeoylquinic acid (6), 4,5-di-O-caffeoylquinic acid (7), jacein (8), centaurein (9) were for the first time isolated from Bidens pilosa. Compounds 2-7 are the major antioxidative constituents in the Bidens pilosa extract.