Arsenic-resistant Chinese hamster ovary (CHO) cells were established by progressively increasing the concentration of sodium arsenite in culture medium. One of the resistant clones, SA7, was also cross-resistant to As(V), Zn, Fe(II), Co, and Hg. The susceptibilities to sodium arsenite in parental CHO cells, revertant SA7N cells, and resistant SA7 cells were correlated with their intracellular glutathione (GSH) levels and glutathione S-transferase (GST) activity. The resistance in SA7 cells was diminished by depletion of GSH in cells after treatment with buthionine sulfoximine. Furthermore, after reexposure of revertant SA7N cells to sodium arsenite, the intracellular GSH levels, GST activity, and resistance to sodium arsenite were raised to the same levels as SA7 cells. These data indicate that the elevation of intracellular GSH levels and GST activity in SA7 cells may be responsible for the resistance to arsenite. A p25 protein, which could be a monomer subunit of GST, accumulated in SA7 cells. In addition, an outward transport inhibitor, verapamil, indiscriminately increased the arsenite toxicity in resistant and parental cells.