Allyl methyl trisulfide (AMT), a constituent of garlic oil, was studied for its effects on glutathione S-transferase (GST) activity and on benzofa. Jpyrene (BP)-induced neoplasia of the forestomach and lungs of female A/J mice. AMT induced increased GST activity in the forestomach, small bowel mucosa, liver, and lung. The forestomach and small bowel mucosa responded to a single low dose of AMT (3.0 pmol) given by oral intubation, whereas liver and lung were less reactive. A dose schedule of two administrations of 15 pmol AMT given 48 hours apart gave close-to-maximum induction in all four tissues and was chosen for investigation of its inhibitory effects. With this dose schedule, AMTproduced an inhibition ofBP-induced neoplasia of the forestomach as shown by a greater than 70% reduction in the number of tumors found at the completion of the experiment. Inhibition of pulmonary neoplasia did not occur. AMT is a member of a new class of naturally occurring chemicals that have the capacity to inhibit chemical carcinogenesis.