The modification potentials of allyl sulfide, germanium and NaCl on the development of preneoplastic hepatic enzyme altered foci were examined in an in vivo mid-term assay system. Rats were initially given a single dose (200 mg/kg), intraperitoneal injection of diethylnitrosamine (DEN) and, starting 2 weeks later, were treated with allyl sulfide, germanium and NaCl at a concentration of 0.5%, 0.05% and 5%, respectively, in the diet for 6 weeks and then killed. All rats were subjected to two-thirds partial hepatectomy at week 3. The modifying potential was scored by comparing the number and area per cm2 of induced glutathione S-transferase placental form-positive (GST-P+) foci in the liver with those of the corresponding control group given DEN alone. Allyl sulfide exerted an obvious inhibition of the development of GST-P+ foci as regards both number and area. On the contrary, germanium showed a significant increase in the number of GST-P+ foci, although the data for area were not altered. NaCl had no modifying effect on their development.