AIM: To establish a arsenic trioxide (As 2O 3 )-resistant leukemic cell line to explore the mechanism of resistance to As 2O 3, and the relationship between the resistant cell line and the multidrug resistance was also investigated. METHODS: The arsenic trioxide (As 2O 3 )-resistant leukemic cell line was established by exposing the cells to the increasing concentration of As 2O 3. MTT assay was used to detect the cytotoxicity. Cell cycle was detected by PI assay. Flow Cytometry was used to detect the P-glycoprotein on the surface of the cells, the intracellular concentration of DNR, and the immuetype of the cells. RESULTS: The cell doublings time and the cell cycle of the arsenic trioxide (As 2O 3 )-resistant leukemic cell line, K562/AS2, is similar to that of K562. The relative resistant fold of K562/AS2 to As 2O 3, DNR, VP16 and Ara-C was 7.4, 2.9, 3.8 and 1.1, respectively. The relative resistant fold of multidrug resistant cell line, K562/ A02, to As 2O 3, DNR, VP16 and Ara-C was 0.8?94?2.5 and 0.9, respectively. The fluorescence of the P-glycoprotein on the surface or of the DNR inside the cells detected was not significantly different between the K562 and the K562/AS2 cell lines. CONCLUSIONS: A cell line, K562/AS2, resistant to clinical achieving level (2 ?mol/L) of As 2O 3 has been established. The relative resistant fold of K562/ AS2 to As 2O 3 is about 7.4 fold to the parent K562 line sensitive to As 2O 3. Partial resistance of K562/AS2 to DNR and VP16 is observed , the mechanism of which is unrelated to the P-gp, the expression product of multidrug resistance gene 1 (mdr1).