Abstract: Objective: To study the effect of arsenic trioxide(As₂O₃) on nucleic acid biosynthesis and apoptosis of melanoma B₁₆ cells.Methods: The nucleic acid biosynthesis in melanoma B₁₆ cells were detected by incorporation of ³H-TdR and ³H-UdR into DNA and RNA,respectively.Apoptosis and necrosis in B₁₆ cells were analyzed by flow cytomertry.Results: As treated with different doses of As₂O₃ in B₁₆ cells the rates of inhibition and apoptosis and necrosis have both dose-dependent and time-dependent manner(P<0.01).Conclusions: As₂O₃ can inhibit nucleic acid biosynthesis and induce apoptosis and necrosis in melanoma B₁₆ cells.The assay showed a dose-response and time-response relationship for inhibition of nucleic acid biosynthesis and apoptosis in B₁₆ cells by As₂O₃.The inhibition was enhanced with the increase of As₂O₃ concentration and prolong of co-culture time.