AbstractAllyl methyl trisulfide (AMT), a constituent of garlic oil, was studied for its effects on glutathione S‐transferase (GST) activity and on benzo[a]pyrene (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 (3.0μmol) given by oral intubation, whereas liver and lung were less reactive. A dose schedule of two administrations of 15μmol 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, AMT produced an inhibition of BP‐induced neoptasia 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.