Hyperacute xenograft rejection of porcine organs by primates is initiated by the binding of recipient natural antibodies to endothelium in the donor organ. We showed previously that human natural antibodies recognize 3 porcine endothelial cell glycoproteins with molecular masses of 115 kDa, 125 kDa, and 135 kDa; we called the glycoproteins gp115/135. We also showed that porcine platelets contain glycoprotein antigens that are the same or very similar to gp115/135 from endothelial cells. The studies reported here were aimed at identifying these porcine platelet antigens and evaluating their potential relevance for the pathogenesis of xenograft injury. The importance of gp115/135 as targets of human natural antibodies was supported by the demonstration that (a) antibodies against porcine platelet gp115/135 are absorbed from the blood of nonhuman primates during the perfusion of a porcine kidney; (b) purified gp115/135 glycoproteins at concentrations < 500 pM, block the binding of human natural antibodies to cultured porcine endothelial cells; (c) the binding of human antibodies to porcine platelet extracts is significantly decreased by removal of gp115 or gp135 from the extracts; (d) antibody binding to gp115/135 initiates the activation of complement. Amino-terminal sequencing of gp115/135 revealed that gp115 has significant homology to human integrin β3 chains, gp125 to human αIIb chains, and gp135 to human α2 chains. Ligand binding properties of the porcine glycoproteins were consistent with the identity of the antigens revealed by amino acid sequencing and molecular weight. Human natural antibodies also recognized a 250 kDa porcine platelet glycoprotein which was found to be homologous to human von Willebrand factor (vWF). Antibodies against vWF in the blood of nonhuman primates are absorbed during ex vivo perfusion of a porcine organ. The identification of gp115/135 as integrins, the functions of which in endothelium might include cell signaling and maintenance of barrier function, provides potential insight into the pathogenesis of the rejection reaction in which these processes are manifestly aberrant. The identification of vWF as a potential target antigen raises several questions, including whether vWF provides one basis for antibody binding to xenogeneic endothelium or whether upon secretion from an endothelial cell vWF might actually block antibody binding.