This study was carried out to investigate the chemopreventive effects of immature plum extracts. The methanol extract of immature plums (plum 1), that are picked at 20-40 days before final harvest, has remarkably inhibited the growth of hepatoma HepG2 cells. The effects of immature plum extracts on hepatotoxicity in benzo(alpha)pyrene (B(alpha)P, carcinogen)-treated mice were investigated. Male ICR mice were pretreated with immature plum extracts (2.5 or 5 g/kg bw/day, for 5 days, i.p.) before treatment with B(alpha)P(0.5 mg/kg bw, i.p., single dose). The activities of serum aminotransferase, cytochrome P450 (CYPs) and the hepatic content of lipid peroxide were increased on B(alpha)P-treatment group than control, but those levels were significantly decreased by the pretreatment of immature plum extracts. The primary CYPs involved in the metabolism and bioactivation of B(alpha)P are CYP1A1. The pretreatment of immature plum extracts inhibited the induction of CYP1A1 expression. The activities of glutathione peroxidase, superoxide dismutase and catalase were decreased by the pretreatment of immature plum extracts more than with B(alpha)P alone. Whereas, the hepatic content of glutathione and glutathione S-transferase activity depleted by B(alpha)P was significantly increased (p > 0.05). These results suggest that immature plum extracts may counteract toxic effects of carcinogens, such as B(alpha)P, and therefore possess the chemopreventive efficacy.