MeOH extract of the leaves of Xanthium strumarium L. were found to have cytotoxic activities against five human tumor cell lines. Cytotoxicity-guided chromatographic fractionation led to the isolation of the ${alpha}-methylene$ containing sesquiterpenes, xanthatin, 8-epi-xanthatin and 8-epi-tomentosin. 8-epi-Xanthatin was found to be far more cytotoxic than 8-epi-tomentosin, which lacks the conjugated enone moiety present in 8-epi-xanthatin.