Peroxiredoxin II (Prx II) is known not only to protect cells from oxidative damage caused by hydrogen peroxide (H202), but also to endow cancer cells with resistance to both H202 and cisplatin and to grant them radioresistance. In this study, we examined whether Prx II antisense could enhance cisplatin-induced cell death. When gastric cancer cells were transfected with various concentrations of Prx II antisense plasmid, pPrxII/AS, and then treated with the same concentrations of cisplatin, Prx II antisense enhanced cisplatin-induced cell death. The combination index (CI) at all doses of the combination was below 1, indicating that Prx II antisense sensitized cisplatin-induced cell death. This synergism was also observed in the cells transfected with a Prx II antisense oligomer. Our present results, therefore, suggest that Prx II antisense would be a very good sensitizer for cisplatin, and that Prx II as a target for chemosensitizers constitutes a promising avenue for future research.