Diallyl sulfone (DASO2) is a metabolite of diallyl sulfide, a compound derived from garlic. The present study investigated the effect of DASO2 as a protective agent against acetaminophen (APAP)-induced hepatotoxicity in mice. Oral administration of DASO2 protected mice against the APAP-induced hepatotoxicity in a dose- and time-dependent manner. When administered 1 hour prior to, immediately after, or 20 minutes after a toxic dose of APAP, DASO2 at a dose of 25 mg/kg completely protected mice from development of hepatotoxicity, as indicated by liver histopathology and serum lactate dehydrogenase levels. Protective effect was observed when DASO2 at a dose as low as 5 mg/kg was given to mice 1 hour prior to APAP administration. Oral administration of DASO2 to mice 1 hour prior to a toxic dose of APAP significantly inhibited the APAP-induced glutathione depletion in the liver. DASO2 treatment also decreased the levels of oxidative APAP metabolites in the plasma without affecting the concentrations of nonoxidative APAP metabolites. In liver microsomes, 0.1 mM of DASO2 caused a 60% decrease in the rate of APAP oxidation to N-acetyl-p-benzoquinone imine, which was determined as glutathione conjugate. This inhibitory effect is mainly due to its inhibition of cytochrome P450 2E1 activity; with an IC50 value equal to 0.11 mM. DASO2 also slightly inhibited the activities of P450s 3A and 1A, with IC50 values > 5 mM. Furthermore, a single oral dose of DASO2 inactivated P450 2E1- and P450 1A-dependent activities in liver microsomes. The results suggest that the protective effect of DASO2 against APAP-induced hepatotoxicity is due to its ability to block acetaminophen bioactivation mainly by the inactivation and inhibition of P450 2E1.