['Human malignant tumors grown as xenografts in immunocompromised animals have been used extensively to study tumor growth and tumor response to therapy. The endothelium functions as an effective barrier between the intravascular space and the tumor cells. In a previous study we used species-specific monoclonal antibodies against endothelial cell adhesion molecules to demonstrate the host origin of the endothelium in xenotransplanted pancreatic islet grafts [Am J Pathol 1995;146:1397-1405]. We now investigated in this study whether the vascular endothelium of different xenografted human malignant tumors expresses mouse (= host)- or human (= graft)-specific CD31 (platelet endothelial cell adhesion molecule, PECAM-1) adhesion molecules.', 'Cultured human prostate, kidney, and colon cancer cells (passages 15-17) were transplanted subcutaneously into 8-week-old athymic nude mice and removed after another 8 weeks. The avidin biotin peroxidase method was utilized on frozen sections to demonstrate that the endothelium of the vasculature of all three human xenografts expressed mouse (= host)-specific CD31, but not human (= graft)-specific CD31.'] Does microvascular endothelium of human tumor xenografts express mouse ( = host ) CD31?