Onosma L. genus (Boraginaceae) comprises of 230 species that is represented by 102 species and 108 taxa in the flora of Turkey. As 50% of the genus is endemic, Anatolia can be considered as the gene center of Onosma species. Phytochemical investigations performed on Onosma genus have led to the identification of various naphtoquinones, alkaloids and phenolic constituents. Biological activity studies on naphthoquinones demonstrated antimicrobial and wound-healing properties as well as cytotoxicity. In this thesis, Onosma taurica var. taurica and O. mollis were taken into cytotoxic activity-guided isolation studies, by MTT using seven human cancer cell lines (DU145, Capan-1, HCC-1937, MCF-7, HeLa, HEPG2, A-459) and a normal cell line (MRC-5), to isolate their bioactive compounds. Additionally, the isolation studies were guided by enzyme inhibition tests towards human Topoisomerases I and II. Six compounds were isolated using chromatographic methods, and the structures of the five of them were elucidated by spectral methods, (1D-, 2D NMR and HR-ESI-MS) as acetylshikonin, shikonin, β-hydroxyisovalerylshikonin, β,β-dimethylacrylshikonin and deoxyshikonin. The isolated compounds showed prominent cytotoxicity with IC50 values ranging from 1.83 to 25 M. β,β-dimethylacrylshikonin was found to be the most cytotoxic compound (IC50: 1.84 μM versus HCC-1937), whereas β-hydroxyisovalerylshikonin on Topoisomerase I and II (each at 25 M dose) exhibited strong inhibitory effects.