Abstract: Objective: To study the influence of As₂O₃ on the proliferation and the reactive oxygen species(ROS) levels in melanoma B₁₆ cells.Methods: B₁₆ cells were treated with 30 μmol/L As₂O₃ alone or together with 500 μmol/L N-acetylcysteine(NAC).The effect of As₂O₃ on B₁₆ cells was determined by MTT assay,and the intracellular ROS level was measured by flow cytometry(FCM).Results: As₂O₃ could obviously inhibit the proliferation of B₁₆ cells and NAC might partially antagonise the apoptosis effect induced by As₂O₃.ROS level in B₁₆ cells started to rise after being incubated with 30 μmol/L As₂O₃ for 1 h(P<0.01),and reached the peak point of at 4 h(P<0.01),and the ROS expression levels decreased slightly at 24 h.The difference was all significant compared with the control group(P<0.01).The increasing of ROS might be partially blocked by NAC(P<0.05 to P<0.01).Conclusions: B₁₆ cells apoptosis induced by As₂O₃ is related to the increase of ROS level.As₂O₃ induces B₁₆ cells apoptosis by accumulation of intracellular ROS level,which may be one of the antitumor processes of As₂O₃.