The superoxide dismutase inhibitor, 2-methoxyestradiol (2ME), has recently been found to have anti-leukemic activity. As both 2-ME and ATO induce apoptosis of leukemic cells through similar mechanisms- mitochondrial damage through increased intracellular ROS, we hypothesized that 2-ME, like ATO, might be particularly effective on acute promyelocytic leukemia (APL). Using MTT assays, we found that the dose of 2-ME required to inhibit 50% of cell growth (IC50=0.25 μM) was 5 times less for NB4 cells than for other types of leukemic cells such as HL-60, U937, K562, THP-1 and KG1a (Fig 1). The exclusive sensitivity correlated with the increase of cellular superoxide and the decrease of cellular glutathione upon treatment but not with the endogenous levels of superoxide or glutathione. 2ME induced apoptosis of NB4 cells through upregulation of pro-apoptotic Bax, and Bak proteins, and induced apoptosis through mitochondrial pathways, shown by the release of cytoplasmic cytochrome C, and cleavage of caspase 9 and 3, and 6. Of note, 2-ME also induced neutrophilic differentiation, confirmed by NBT assays, and the expression of CD11b. We showed that, although 2-ME did not affect the PML/RARα transcription, it decreased the levels of PML/RARα in the cells. Like ATO, 2-ME specifically inhibited the activation of NF-κB in NB4 cells but not in HL-60 cells, shown by the EMSA assay (Fig 2). The effectiveness of 2-ME on NB-4 cells could also be observed in both all-trans retinoic acid (ATRA)- resistant and ATO-resistant cells. Furthermore, a combination of 2-ME with ATO was more effective on NB4 cells than either agent alone. Therefore, 2-ME could be potentially useful for eradication of APL or as a salvage agent for ATRA-and ATO-resistant APL.