BACKGROUND: Present studies on the reversal of multi-drug resistance (MDR) are almost entirely limited to in vitro systems, and are seldom carried out in vivo. In order to study MDR under the in vivo situation, a MDR cell strain was used to establish a model in nude mice via orthotopic implantation directed by B-ultrasonography. This model is expected to provide a good platform for evaluating strategies to reverse the phenomenon of primary hepatocellular carcinoma (HCC) MDR. METHODS:An orthotopic MDR1 hepatoma was obtained by injecting the cell lines HepG2 and HepG2/ADM subserosally into the liver of nude mice (10 control and 20 MDR mice). The injections were made under B-ultrasonographic direction. Ultrasonography and laparotomy were used to assess tumor growth, and the long chain PCR technique was applied to sequence the MDR1 gene from multi-drug resistant human HCC cells, HepG2/ADM, and corresponding implanted tumor tissues. Furthermore, MDR1 mRNA and P-gp protein expression were evaluated by reverse transcriptase-polymerase chain reaction (RT-PCR), Western blotting, and immunohistochemical methods. RESULTS: The success rates of tumor implantation and induction of MDR were 100% (30/30) and 95% (19/20), respectively. The 3.8kbp MDR1 gene band was detected from the HepG2/ADM cell line and the correspondingimplanted tumor tissues, and the MDR1 gene sequence coincided with that reported in GenBank. The expressions of MDR1 mRNA and P-gp protein in MDR mice were signiifcantly higher than those in the control group. There was a signiifcant difference in P-gp protein expression between MDR and control mice. CONCLUSION:A MDR model has been successfully established in nude mice via orthotopic implantation of multi-drug resistant human HCC cells directed by ultrasonography.