Sesquiterpenes are natural chemical compounds that are C-15 terpenoids by their chemical structure. They appear as hydrocarbons or oxidized form products as alcohols, aldehydes, acids or lactones in nature. They are important ingredients of plant extracts and essential oils that have significant application in medical, fragrant preparations or soaps. Sesquiterpene lactones own important biologically active properties that can be very useful in many disease treatments. They can have anti-inflammantory, antitumor, antimicrobial effects on central nervous and cardiovascular system as well as other biological effects. Of biological activities, cytotoxic, antitumor activity is significant on human cells. Cytotoxic activity is realted to α,β-unsaturated carbonyl structure such as α-methylene-γ-lactone or to α,β- unsaturated cyclopentanone or conjugated ester. Sesquiterpene lactones cynaropicrin and costunolide can be isolated from plants of Asteracea family, especially from plants of its genus Centaurea. Their cytotoxic activity is well known and proven in numerous studies. In this paper, the cytotoxic activity of cynaropicrin and costunolide sesquiterpene lacones was tested on human fibroblast cells. Cell viability was determined by MTT assay. We have shown that it is necessary to test higher concentrations of cynaropicrin of 2,890 μM and costunolide of 21,500 μM in order to achieve cell growth inhibition values lower than 50%. Cynaropicrin tests have shown that lower concentrations have positive effect on fibroblast cell growth and further research is needed in order to determine what the effect and mechanism of activation is.