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A crude aqueous acetone extract of Osbeckia aspera Blume (Melastomataceae), a plant from Sri Lanka used traditionally to treat liver disease, was fractionated by column and preparative paper chromatography, and the fractions were analyzed by high-performance liquid chromatography (HPLC) using diode array and mass spectrometric detection. Phenolic acids (gallic, protocatechuic, and ellagic acid), flavonol glycosides [quercetin 3-O-β-galactopyranoside, quercetin 3-O-β-glucopyranoside, kaempferol 3-O-β-glucopyranoside, and kaempferol 3-O-(6''-O-p-coumaroyl-β-glucopyranoside) (tiliroside)] and flavonol aglycones (quercetin and kaempferol) were identified by comparison of their retention times, UV and MS spectra with those of authentic standards. Five compounds from a methanol extract were identified by NMR spectroscopy as the flavonol glycosides, quercetin 3-O-(3''-O-acetyl-β-galactopyranoside) and kaempferol 3-O-[2'',6''-di-O-(E,E)-p-coumaroyl-β-glucopyranoside], and the norsesquiterpenoids 6,9-dihydroxy-4,7-megastig-madien-3-one, 9-hydroxy-4,7-megastigmadien-3-one and 9-hydroxy-4-megastigmen-3-one. A crude water extract, 50% acetone extract and fractions from this extract, a 100% methanol extract, and three of the phenolic acids in the fractions were tested for in vitro hepatoprotective activity against bromobenzene and 2,6-dimethyl-N-acetyl p-quinoneimine toxicity to HepG2 liver-derived cells. The crude water extract showed protective activity against both liver toxins, whereas the fractions and compounds were more protective against 2,6-dimethyl-N-acetyl p-quinoneimine than bromobenzene. Of the three phenolic acids present in the extracts that were tested, gallic and protocatechuic acids were more active at protecting the liver cells from the two toxic compounds than ellagic acid. [ABSTRACT FROM AUTHOR]