The cytotoxicity of two protoberberine alkaloids: berberine and lincangenine, their 8-hydroxy-7,8-dihydro-derivatives and tetrahydroprotoberberine:thaicanine, was evaluated. The cellular responses through the [3-(4,5-dimethylthiazol- 2-yl)-2,5-diphenyl-2H-tetrazolium bromide] (MTT) method were measured in Hela (uterus carcinoma), SVKO3 (ovary carcinoma), Hep-2 (larynx carcinoma), primary culture from mouse embryon, and human fibroblast cells at the concentration: 10–1000 ppm (mg:ml) for 24 h. Berberine showed the highest cytotoxicity among the compounds tested, giving LC50 values for all cell lines at the concentration of 10 ppm. The results indicated that the cytotoxicity was notably decreased by structural changes, i.e. by modulation of the planarity caused by the introduction of hydroxyl group at C-8 and concomitant saturation of double bond between N-C8 in protoberberine molecules. In the case of berberine, the cytotoxic effect changed from 98.8 (berberine) to 39% for 8-hydroxydihydroberberine at the concentration of 100 ppm in Hela cells line. The same effect was observed with lincangenine and 8-OH-lincangenine (cytotoxicities 70 and 25%, respectively, at 1000 ppm in SVKO3 cells). On the other hand, these compounds showed a low selectivity for the different human cancer cell lines tested.