Human hepatocyte growth factor (hHGF) is a potent mitogen, motogen and/or morphogen in various organ systems including the liver, mammary gland, bone marrow, lungs, and gastrointestinal tract. c-met, a proto-oncogene is the cellular receptor for hHGF. In this study, I report the development and characterization of two transgenic animal models: human hepatocyte growth factor/severe combined immunodefeciency (hHGF/SCID) and human hepatocyte growth factor/recombinase activating gene I mice (hHGF/RAG1). SCID and RAG1 mice were serially backcrossed with TG.AH2 (a transgenic mouse overexpressing human HGF), in order to select for both the immunodeficiency trait and for expression of serum human HGF production. Both models demonstrated characteristic liver enzyme profiles pre and post partial hepatectomy, specific responses to whole body irradiation, serum hHGF concentrations of up to 250 ng/ml, and an increased Bromodexoyuridine labeling index (up to 4 x control) in the liver. There were histopathologic changes in the liver: severe, diffuse hepatocellular hypertrophy with pleiotrophic nuclei, numerous bi- and tri-nucleated hepatocytes, and cribriform-shaped nuclei. Up to 15% of the hHGF/SCID animals demonstrated tumors of epithelial and mesencyinal origin. Mammary adenocarcinoma, well-differentiated papillary adenocarcinoma, multi-organ lymphoma with metastasis, and hemorrhagic ovarian cyst with dysplastic epithelium also were seen and characterized. Non-neoplastic changes included diffuse lymphoplasmacytic interstitial pneumonia, interstitial nephritis and hypoplastic bronchi-associated lymphoid tissue. We transplanted a human liver epithelial cell line intrasplenically and subcutaneously in the presence of gelfoam. Histological and immunohistochemical techniques identified hepatocytes in the spleen, liver and gelfoam sponge after 2, 7 and 14 days post-transplantation, and the cells were detectable in the gelfoam sponge up to 60 days post-transplantation. I have developed and characterized an immunodeficient transgenic mouse model expressing high level of serum hHGF. Transformed human hepatocytes were transplanted into the model, and they were sustained for at least 60 days post-transplantation. The model should be useful for cellular and molecular biology studies of human hepatocytes, human gene therapy and infectious diseases of human hepatocytes.