It has been shown recently in China that arsenic trioxide (As₂O₃) is a very effective treatment for acute promyelocytic leukemia (APL). APL patients resistant to all-trans retinoic acid (ATRA) and conventional chemotherapy can still respond to As₂O₃. In this study, we addressed the possible cellular and molecular mechanisms of this treatment by using NB₄ cells as a model. The results show that: (1) As₂O₃ triggers relatively specific NB₄ cell apoptosis at micromolar concentration, as proved by morphology, histogramic related nuclear DNA contents, and DNA gel electrophoresis. (2) As₂O₃ does not influence bax, bcl-x, c-myc, and p53 gene expression, but downregulates bcl- 2 gene expression at both mRNA and protein levels. (3) As₂O₃ induces a significant modulation of the PML staining pattern in NB₄ cells and HL-60 cells. The micropunctates characteristic of PML-RARα in NB₄ cells disappear after treatment with As₂O₃, whereas a diffuse PML staining occurs in the perinuclear cytoplasmic region. In addition, a low percentage of untreated NB₄ cells exhibits an accumulation of PML positive particles in a compartment of cytoplasm. The percentage of these cells can be significantly increased after As₂O₃ treatment. A similar PML staining pattern is observed in apoptotic cells. (4) ATRA pretreatment does not influence As₂O₃-induced apoptosis. These results suggest that induction of cell apoptosis can be one of the mechanisms of the therapeutic effect of As₂O₃. Moreover, this apoptosis induction occurs independently of the retinoid pathway and may be mediated, at least partly, through the modulation of bcl-2, as well as PML-RARα and/or PML proteins.