Among dental metals, copper showed the highest cytotoxicity against human oral squamous cell carcinoma and human submandibular gland carcinoma cells, followed by palladium-alloy, gold and silver. Normal human cells (gingival fibroblast, pulp cells, periodontal ligament fibroblast) were relatively resistant to these metals. The palladium-alloy failed to induce internucleosomal DNA fragmentation, a biochemical hallmark of apoptosis, in human promyelocytic leukemic HL-60 cells. The cytotoxic activity of the palladium-alloy was significantly reduced by a non-cytotoxic concentration of N-acetyl-L-cysteine, or more efficiently by sodium ascorbate. However, higher concentrations of sodium ascorbate enhanced the cytotoxic activity of palladium-alloy. ESR spectroscopy showed that the palladium-alloy enhanced the intensity of ascorbate radical, suggesting the possible interaction between metals and antioxidants. All metals, except copper, did not significantly affect the generation of superoxide anion (by hypoxanthine-xanthine oxidase reaction), hydroxyl radical (by Fenton reaction) and nitric oxide (from NOC-7 in the presence of C-PTIO). These data demonstrate for the first time that antioxidants modify the biological activity of dental metals.