Objective: To further understand the possible mechanisms of arsenic sulfide (realgar) in the treatment of acute promyelocytic leukemia (APL). Methods: All-trans retinoic acid (ATRA)-susceptible APL cell line (NB4 cells) and ATRA-resistant APL cell line (MR2 subclone) were used as models in vitro. At various times after incubated with various concentrations of realgar, NB4 and MR2 cells were observed by cell viability , cell proliferation and cell morphology; cell cycle and the expression of Annexin V were assayed by flow cytometry. Results: Cell viability and proliferation of NB4 and MR2 cells were inhibited after the treatment, to some extent, in a dose and time dependent manner. 177 - 711g/L of realgar treated NB4 and MR2 cell presented morphologically some features of apoptotic cells such as intact cell membrane, chromatin condensation and nuclear fragmentation, apoptosis body could be found by electron microscopy as well. Sub-Gl cells and cell cycle arrest were observed by flow cytometry. The proport Objective: To further understand the possible mechanisms of arsenic sulfide (realgar) in the treatment of acute promyelocytic leukemia (APL). Methods: All-trans retinoic acid (ATRA) -sensitive APL cell line (NB4 cells) and ATRA-resistant APL cell line (MR2 subclone) were used as models in vitro. At various times after incubated with various concentrations of realgar, NB4 and MR2 cells were observed by cell viability, cell proliferation and cell morphology; cell cycle and the expression of Annexin V were assayed by Flow cytometry. Results: Cell viability and proliferation of NB4 and MR2 cells were inhibited after the treatment, to some extent, in a dose and time dependent manner. 177 - 711g / L of realgar treated NB4 and MR2 cell presented morphologically some features of apoptotic Cells such as intact cell membrane, chromatin condensation and nuclear fragmentation, apoptosis body could be found by electron microscopy as well. Sub-Gl cells and cell cycle arrest were observed by flow cyt ometry. The proport