We show that Japanese-style fermented soy sauce (shoyu) contains anticarcinogenic activity. Female ICR mice were fed a semipurified diet containing soy sauce (0-30%). Two weeks later a regimen consisting of 4 doses of benzo(a)pyrene (1 dose/week p.o. for 4 weeks) was begun to initiate forestomach neoplasia. Twenty-three weeks after the first intubation the animals were sacrificed, and forestomach neoplasms were counted and histologically confirmed. Soy sauce produced a significant dose-dependent reduction in forestomach neoplasms, which appeared to be maximal when soy sauce constituted 20% of the diet. Exposure to nitrite (0-500 ppm through drinking water) neither enhanced nor diminished the anticarcinogenic effect of the dietary soy sauce. Soy sauce was found to contain antioxidant activity which may be related to the observed anticarcinogenic effect. Contrary to expectations, mouse forestomach ornithine decarboxylase activity was induced by soy sauce. This appeared to be due at least in part to the relatively high sodium chloride content of soy sauce.