Hepatocarcinogenesis in woodchucks that are persistently infected with woodchuck hepatitis virus (WHV) follows a progressive course characterized by foci of altered hepatocytes, benign neoplastic nodules and finally hepatocellular carcinoma (HCC). In situ hybridization studies have demonstrated that insulin-like growth factor II (IGF-II) is expressed in most HCCs in woodchucks but that the patterns of expression are variable from tumor to tumor. In some cases, expression of IGF-II is high throughout the tumor, and in others expression is limited to the growing edge of the tumor. IGF-II expression is also activated in focal groups of cells in neoplastic nodules. The major IGF-II mRNA in nodules and HCCs is a 3.4 kb transcript corresponding to one of two IGF-II RNAs in fetal woodchuck liver. A single 15 kDa IGF-II polypeptide accumulates in the perinuclear cytoplasm of hepatocytes in fetal woodchuck liver, neoplastic nodules and HCCs. Thus IGF-II expression in woodchuck liver is reactivated in lesions which are believed to be the precursors of HCC and continues to be expressed as HCCs develop.