The anti-initiating properties of allyl sulfides on rat liver carcinogenesis induced by N-nitrosodiethylamine (NDEA) or aflatoxin B1 (AFB1) were evaluated by using a three-step medium-term hepatocarcinogenesis assay. Diallyl sulfide (DAS) or diallyl disulfide (DADS) was added to the diet of rats (2 g/kg) for three weeks, during which NDEA or AFB1 was administered by intraperitoneal injection. The rats were submitted later to eight days of 2-acetylaminofluorene administration and to two-thirds hepatectomy, then to phenobarbital administration. After eight weeks, liver preneoplastic foci expressing the placental form of glutathione S-transferase were detected. The results show that DAS and DADS strongly reduced the number and the size of preneoplastic foci initiated by NDEA and AFB1, but especially by AFB1; DADS is more efficient than DAS. Most likely, the inhibition of the first step of hepatocarcinogenesis by allyl sulfides is related to the modulating effects that these compounds exert on the enzymes involved in activation and/or detoxication of the carcinogens. Our study demonstrated the chemopreventive potencies of dietary allyl sulfides in liver carcinogenesis induced by two potent hepatic carcinogens.