Abstract： Objective To investigate the effect of arsenic trioxide (As2O3) and buthionine sulfoximine (BSO) on inhibiting P-glycoprotein expression in multidrug-resistant cell-K562/ADM cell. To compare the effect of As2O3 and the combined group .To determine the effect of intracellular glutathione content on the arsenic effect. Methods To investigate the effect of the arsenic group (0.5 μmol/L, 2.0 μmol/L, 5.0 (μmol/L) and/or BSO (100 μmol/L) on K562/ADM cell. Intracellular GSH contents were measured using glutathione assay kit by spectrophotometry.P-gp expression were determined by flow cytometry. Mdr-1mRNA expression were directed by semi-quantitative RT-PCR. Results P-gp expression and mdr-1mRNA expression were inhibited in 24 hours in the combination of clinic dose arsenic group (0.5 (μmol/L, 2 μmol/L)and 880(100 μmol/L). In 48 hours, the mdr-1mRNA depressive effect of the combination group (clinic dose arsenic group) was obviously stronger than high dose arsenic group. In 72 hours, the P-gp depressive effect of the combination group (clinic dose arsenic group) was obviously stronger than high dose arsenic group.Conclusion The combination of clinic dose arsenic and BSO inhibit obviously P-gp expression and mdr-1mRNA expression in K562/ADM cell.