Seeding of heart valve prostheses with human umbilical vein endothelial cells (HUVEC) and human saphenous vein endothelial cells (HSVEC) has been used to create a viable valve surface and improve valve performance. HUVEC and HSVEC have been used as a model of endothelial antigenicity, but the antigenicity of valve endothelium is less well known. Thus, we studied expression of blood group antigens by human valvular endothelium, HSVEC and HUVEC. Human aortic and mitral valves and myocardial tissue were obtained from explanted hearts of patients undergoing heart transplantation (blood groups A and O) or valve replacement (blood group B). After tissue fixation, sections were stained with anti-A (blood group A), anti-B (blood group B), and anti-H (blood group O) antibodies. Human umbilical cords were freshly harvested postpartum, and human saphenous veins were from patients undergoing coronary bypass grafting; both were fixed and stained to detect ABO antigens. Endothelial preservation was confirmed by staining with anti-CD 31 monoclonal antibody. All sections were examined by light microscopy. CD 31 staining showed vascular and valve endothelial preservation. Human umbilical cords, human saphenous vein and myocardium showed strongly expressed A, B and H blood group antigens on vascular endothelium, but A, B and H antigens were not detected on valvular endothelium. In conclusion, valve endothelial cells appear to be specialized and do not express ABO antigens. Due to the strong expression of A, B and H antigens by HUVEC and HSVEC, blood group cross-matching should be considered for non-autologous endothelialization of valve prostheses.