Endothelial cell activation with accompanying vascular inflammatory changes is considered central to the experimental manifestations of both hyperacute and delayed xenograft rejection responses. Natural xenoreactive antibodies directed at alpha-galactosyl residues of xenogeneic glycoproteins and glycolipids, with associated complement activation via the classical pathway, are considered major immediate mediators of graft endothelial cell injury in the clinically relevant discordant swine to primate combinations. In delayed xenograft rejection processes, where recipients are treated prophylactically to ameliorate these initial events, activation of infiltrating mononuclear phagocytes and natural killer cells are associated with ongoing endothelial cell activation processes, procoagulant generation and vascular thrombosis. Allograft hyperacute rejection is observed when vascularised organs are transplanted to sensitized individuals with high levels of cytotoxic antibodies. Less dramatic forms of humoral allograft rejection (termed accelerated or vascular rejection) and the more common cell-mediated endothelialitis are associated with significant graft damage. Endothelial cell activation is also linked with graft preservation injury, forms of chronic rejection and delayed graft loss. Experimental work is currently being directed at the control of hyperacute rejection, the close understanding of endothelial cell thromboregulation in both transplanted xeno- and allografts and the development of novel therapeutic agents including gene therapy and the possible use of organs from transgenic animals.