The reactive intermediates formed during the metabolism of therapeutic agents, toxicants and carcinogens by cytochromes P450 are frequently capable of covalently binding to tissue macromolecules and causing tissue damage. It has been shown that YH439, a congener of malotilate, is effective in suppressing hepatic P450 2E1 expression. The present study was designed to further establish the mechanistic basis of YH439 protection against toxicant by assessing its effects against chemical-mediated potentiated hepatotoxicity. Retinoyl palmitate (Vit-A) pretreatment of rats for 7 days substantially enhanced carbon tetrachloride hepatotoxicity, as supported by an ~ 5-fold increase in serum alanine aminotransferase (ALT) activity, as compared to CCl₄ treatment alone. The elevation of ALT activity due to Vit-A was completely blocked by the treatment of GdCl₃ a selective inhibitor of Kupffer cell activity. Concomitant pretreatment of rats with both YH439 and Vit-A resulted in a 94% decrease in Vit-A-potentiated CCl₄ hepatotoxicity. YH439 was also effective against propyl sulfide-potentiated CCl₄-induced hepatotoxicity. Whereas propyl sulfide (50 mg/kg, 7d) enhanced CCl₄-induced hepatotoxicity by >5-fold, relative to CCl₄ treatment alone, concomitant treatment of animals with both propyl sulfide and YH439 at the doses of 100 and 200 mg/kg prevented propyl sulfide-potentiated CCl₄ hepatotoxicity by 35% and 90%, respectively. Allyl sulfide, a suppressant of hepatic P450 2E1 expression, completely blocked the propyl sulfide-enhanced hepatotoxicity, indicating that propyl sulfide potentiation of CCl₄ hepatotoxicity was highly associated with the expression of P450 2E1 and that YH439 blocked the propyl sulfide-enhanced hepatotoxicity through modulation of P450 2E1 levels. Propyl sulfide- and CCl₄-induced stimulation of lipid peroxidation was also suppressed by YH439 in a dose-related manner, as supported by decreases in malonedialdehyde production. The role of P450 2E1 induction in the potentiation of CCl₄ toxicity and the effects of YH439 were further evaluated using pyridine as a P450 2E1 inducer. Pyridine pretreatment substantially enhanced the CCl₄ hepatotoicity by 23-fold, relative to CCl₄ alone. YH439, however, failed to reduce the pyridine-potentiated toxicity, suggesting that the other form(s) of cytochroms P450 inducible by pyridine, but not suppressible by YH439 treatment, may play a role in potentiating CCl₄-induced hepatotoxicity. YH439 was capable of blocking cadmium chloride-induced liver toxicity in mice. These results demonstrated that YH439 efficiently blocks Vit-A-enhanced hepatotoxiciy through Kupffer cell inactivation and that the suppression of P450 2E1 expression by YH439 is highly associated with blocking of propyl sulfide-mediated hepatotoxicity.