Inula aucheriana n-hexane, CHCl3 and MeOH extracts were evaluated for their anti-proliferative activity against HepG-2, MCF-7, MDBK and A-549 cells. The CHCl3 extract exhibited cytotoxic activity to the above cell lines with IC50 values of 13.5, 23.4, 10.5, and 26.9 microg/mL, respectively. The sesquiterpene lactone britannin was isolated from the above extract. This was further evaluated in the MTT assay to demonstrate strong cytotoxicity to the mentioned cell lines (IC50: 2.2, 5.9, 5.4, and 3.5 microg/mL, respectively), and the apoptotic inducing properties of britannin were evaluated on human breast adenocarcinoma (MCF-7) cells through the terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay.