Metastasis is a highly selective process during which cancer cells colonize secondary organs in distinct patterns. Organ colonization is initiated by the arrest of blood-borne tumor cells in selected blood vessels of the secondary target organ. This event appears to be regulated by organ-specific adhesion molecules expressed on the lumenal surface of vascular endothelial cells (ECAMs). To study the regulatory role of ECAMs in metastasis, we rely in our experiments on an essay system that circumvents the cumbersome isolation and culture of phenotypically stable, microvascular endothelial cells and, instead, use large vessel endothelial cells [e.g., bovine aortic endothelial cells (BAEC)] modulated with organ-specific extracellular matrix to assume phenotypic traits of the organ from which the matrix has been derived. This system has been successfully used in generating monoclonal antibodies (mAbs) that crossreact with a lung-specific B16 melanoma cell-binding mouse ECAM (Lu-ECAM-1) and, thus, constitute the foundation for our studies on the vascular arrest of blood-borne cancer cells. The goal of our studies is to purify receptor-ligand pairs from lung (or liver)-specific endothelium and lung (or liver)-metastatic tumor cells and to clone their respective genes. Tissue distribution as well as translational and transcriptional regulation of these molecules on the endothelial and tumor cell surfaces are analyzed under various microenvironmental conditions (e.g., various organ-specific extracellular matrices). Further studies are directed to determine ECAM tumor ligand distribution in primary tumors and to compare such distributions with eventual metastatic spread and, finally, to assess adhesion of tumor cells to endothelial cell monolayers or various densities of purified, immobilized ECAM (or combinations of different ECAMs) under conditions of fluid flow and shear stress. The ultimate goal of these studies is to exploit our understanding of the molecular mechanisms of tumor cell/endothelial cell interactions for the development of new anti-metastatic treatment strategies.