The development of resistance to imatinib mesylate may partly depend on high Bcr-Abl-expression levels. Arsenic trioxide (ATO) has Bcr-Abl suppressing activity in vitro. Here we investigated means to improve ATO activity in CML by modulating cellular glutathione (GSH), a key regulator of ATO-activity in malignant disease. Our studies demonstrate that depletion of cellular glutathione using dl-buthionine-[S,R]-sulfoximine (BSO) enhances ATO activity against CML cells. GSH-depletion promotes enhanced Bcr-Abl specific activity of ATO through avid repression of Bcr-Abl protein levels and total cellular Bcr-Abl activity. These data provide a rationale for the clinical development of optimized ATO-based regimens through incorporation of GSH-modulators in CML treatment.