Cytotoxic activity of artemisinin and derivatives in the presence and absence of holo-transferrin and expression of genes involved in resistance of cancer cells were investigated in human cholangiocarcinoma (CL-6) and hepatocarcinoma (Hep-G2) cell lines in vitro. After incubation with the test drugs and 5-fluorouracil (5-FU) cytotoxicity was asessed by MTT assay. RNA was extracted after 24 hour exposure to holo-transferrin for invesstigation of the expression of transferrin receptor 1 (TDR1), multidrug resistance 1 (MDR1), multidrug resistance protein 1 (MRP1), multidrug resistance protein 2 (MRP2), and multidrug resistance protein 3 (MRP3). The median IC₅₀ of artemisinin, artesunate, artemeter, dihydroartemisinin and 5-FU were as follows: CL-6: 339, 131, 354, 75, and 377 microM, respectively; Hep-G2: 268, 50, 233, 29, and 1,380 microM. Exposure to holo-transferrin had no influence on sensitivity of either cell line to artemisinin derivatives, but resulted in a 3-fold increase in the expression of TR1 and MDR1, and a 2-fold increase in the expression of MRP1 and MRP2 in CL-6 cells. With Hep-G2, a 3-fold increase in the expression of MDR1 and MRP3 and a 2-fold increase in expression of MRP2 were observed. Dihydroartemisinin exhibited the most potent cytotoxic activity against both cell lines and holo-transferrin caused different patterns of expression of resistance-associated genes.