Abstract. In the present study, in order to clarify the preventive mechanism of N-acetyl-L-cysteine (NAC) on arsenite-induced apoptosis in U937 cells, which lack functional p53, the cytotoxicity among U937 cells [monocytes and 12-O-tetradecanoylphorbol-13-acetate (TPA)-treated macrophages] receiving NAC treatment at different times post arsenite treatment was examined. TPA-treated macrophages were more resistant to arsenite-induced apoptosis than monocytes, which may be associated with the induction of Bcl-2 expression. Pretreatment with 20 mM NAC prior to arsenite exposure suppressed apoptosis up to 75% in the monocytes and 100% in the macrophages. However, 6-h NAC pretreatment and subsequent washing out of NAC from the culture medium prior to arsenite treatment did not inhibit the arsenite-induced apoptosis. Post-treatment by NAC up to 1 h following arsenite exposure almost completely inhibited the cytotoxic effects of arsenite in U937 monocytes and macrophages. The results of the current study indicate that the preventive mechanism of NAC on arsenite-induced apoptosis in U937 monocytes and macrophages mainly involves chelation of arsenite in culture medium.