Abstract: Objective: To establish an animal model using nude mice injected with K562/A02, a leukemia cell line with, stable multiple drug resistance (MDR), in order to study the mechanism of drug resistance of tumors in vivo and to screen for drugs that can reverse this effect. Methods: K562/ AO2 cells, classically MDR, were subcutaneously injected in the right flank of nude mice, and the parent K562 cells were subcutaneously injected in the left flank of the mice. Each injection contained 1×10⁷cells. The conditions and growth characteristics of the tumors were observed, and the sensitivity to chemotherapy of the injected tumor cells and in vitro culture cells were compared. Tumors arising from the injected cells were also analyzed using RT-PCR and immunohistochemical staining Results: a) The tumorigenesis rate of K562/A02 cells in nude mice was 100%. In 6 to 9 days, the tumors reached a volume of more than 100 mm³ and there was no apparent difference in the growth rate between the drug resistant and the sensitivetumors; b) For adriamycin, the IC₅₀ for K562/A02 injected cells and the in vitro cultured primary cells was 185.24μg/ml and 235.25μg/ml, respectively. The IC₅₀ for K562/A02 injected cells and the in vitro cultured primary cells was 3.07μg/ml and 3.26μg/ml, respectively. The growth of the sensitive K562 cells was significantly reduced in the mice treated with driamycin, but the drug had little effect on the growth of drug resistant K562/A02 cells injected into the mice; c) The K562/A02 tumor cells in nude mice had mdr1/Pgp expression, yet the K562 tumor cells in nude micehad no such expression. Conclusion: A model for multiple drug resistant tumors in nude mice that maintains the drug resistance seen in vitro has been established using K562/A02 cells. This model has potential for research on the mechanism of drug resistance and its reversal.