The fresh spiny flowers of Centaurea solstitialis ssp. solstitialis (CSS) are used for the treatment of peptic ulcers in Turkey. Ethanol (80%) extract of CSS exhibited significant anti-ulcerogenic effect on the ethanol-induced ulcerogenesis model in rats. The ethanol extract was further fractionated by successive solvent extractions with n-hexane, chloroform, ethyl acetate and n-butanol. All fractions showed significant anti-ulcerogenic activity, however, the effect of the chloroform fraction was found to be more prominent with 99.5% ulcer inhibition. Bioassay-guided fractionation yielded sesquiterpene lactones as the active components. The main components responsible for the activity of the chloroform fraction were determined as chlorojanerin and 13-acetyl solstitialin A which were elucidated by HR-ESI and H-1, C-13 and 2D NMR spectroscopic techniques. (C) 2004 Elsevier Ireland Ltd. All rights reserved.