As₂O₃ is cytotoxic to HL60 and U266 cells. HL60 (A) and U266 (C) cells were treated with 1, 2, 3, 5, and 10 μMAs₂O₃ for 24 (○) and 48 (•) hours and viability was determined by trypan blue exclusion. Cloning efficiency of HL60 (B) and U266 (D) cells treated with 1 to 3 μM As₂O₃ or were left untreated. (A-D) Data are expressed as the mean ± SD of 3 independent experiments.