The aim of this study was to elucidate the characteristics of glutathione S-transferase (GST)-activating compounds from medicinal plants. Among 265 kinds of medicinal plants, Phellodendron amurense showed the highest GST activity at 174.8%. The GST-activating compound of P. amurense was maximally extracted when treated with distilled water at 30 degrees C for 12 h. The compound was purified by ultrafiltration, Sephadex G-10 gel filtration chromatography, and reverse-phase HPLC. The purified GST-activating compound from P. amurense was a novel tetrapeptide with an amino acid sequence of Ala-Pro-Trp-Cys and its molecular weight was estimated to be 476 Da. It also displayed a clear detoxicative effect in 1-chloro-2,4-dinitrobenzene treated mice at a dosage of mg/kg body weight.