Malign tumours are, along with cardiovascular diseases, the main cause of death of human population. Harmful consequences and development of drug resistance are the main problems of successful chemotherapies. There is a big need for more efficient and less harmful substances, potential chemotherapeutics. In this thesis, biological effect of sesquiterpene lactone isolated from Croatian endemic plant Centaurea ragusina L. (dubrovačka zečina) was examined. Biological effect of sesquiterpene lactone ragusinin on human cervical carcinoma (HeLa) cell line was examined using colorimetric assay MTT (3- (4,5-dimetyilthiazol-2-yl)-2,5-diphenyltetrazoli bromide) and the half-maximal inhibitory concentration (IC50) of the ragusinin was obtained at 2,2 μM. We also analysed the cell cycle progression by propidium iodide staining using flow cytometry. The obtained data indicated ragusinin as a potent inducer of apoptotic cell death and as a trigger for increase in G2 phase population. The ability of ragusinin to induce apoptosis of HeLa cells was examined by Annexin V-FITC Assay using flow cytometry. The cleavage of caspase 3 and PARP was not detected indicating that ragusinin triggers caspase-independent cell death. Using specific precursors and inhibitors of glutathione synthesis and of other molecules involved in glutathione metabolism, it was shown that glutathione is involved in the mechanism of ragusinin cytotoxicity inhibition probably through detoxification mechanism, and not by stabilization of oxidative-redox system in the cell.