The effects of ethyl alcohol and pig serum administration 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). Two weeks later, treatment was started with 10% ethanol + 10% sucrose solution, 10% sucrose solution, or tap water as drinking water for 6 weeks with or without intraperitoneal injection of porcine serum twice a week. All rats were subjected to a two-thirds partial hepatectomy at week 3. The modification potentials were evaluated by comparing the number and area per cm2 of glutathione S-transferase placental form-positive (GST-P+) foci in the liver of each group. As a result, ethanol significantly enhanced the development of GST-P+ foci. Unfortunately, the porcine serum injection produced no hepatic fibrosis and no significant alteration in GST-P+ foci.