Ultrafiltration, ion-exchange chromatography, and gel filtration chromatography were used to purify anti-oxidant peptides from hydrolysates of soybean protein isolates. The <1-kD peptides were found to exhibit much higher anti-oxidant activity as compared to larger ones. Also, the alkaline peptide fractions were shown to have stronger anti-oxidant capacity than acidic and neutral peptides. Interestingly, an anti-oxidant tripeptide, Ser-Phe-Val (352.4 Da), was identified by reversed-phase high-performance liquid chromatography (RP-HPLC) connected online to electrospray ionization mass spectrometry. The tripeptide was further prepared by fluorenylmethoxycarbonyl (Fmoc) solid-phase synthesis and was found to have a dose-dependent protective effect against hydrogen peroxide (H2O2)-induced injuries in rat adrenal pheochromocytoma (PC12) cells.