Even though the liver is a relevant metastatic site for several human malignant tumors, mechanisms or organ-specific metastasis to the liver remain largely unknown. In the following paper we summarize the results obtained with different murine model systems which have been set up to elucidate the above mechanisms, and describe our own results with two murine models: the F9 teratocarcinoma and the B16 melanoma. While the F9 teratocarcinoma model system underscores the roles of both adhesion and growth stimulation in the target organ, the B16 melanoma model strengthens the relevance of paracrine growth stimulation. Moreover, B16 melanoma cells selected in vivo for increased liver colonization ability appear to depend on cell-to-cell contact with hepatocytes in order to gain efficient growth stimulation. When we next tried to identify the molecule(s) responsible for the growth effect in a liver plasma membrane extract, we found that such activity was mediated by two closely related protein bands. These turned out to be two different forms of transferrin (Tf), one of which is specifically present on the hepatocyte surface. Moreover, when we analyzed the different B16 lines for the expression of c-Met[the receptor for the hepatocyte growth factor-scatter factor (HGF/SF)], we found that liver-specific LS9 had more of this protein than lung-specific F10 or parental F1, suggesting a role for HGF/SF in liver colonization by B16 melanoma cells.