Background. Hyperacute rejection of vascularized discordant xenografts can now be effectively managed. However, acute vascular rejection (AVR) then ensues, resulting in graft destruction, coagulopathy, or both within weeks. The aim of this study was to determine associations between humoral responses to the xenograft and the induction of AVR, coagulopathy, or both. Methods. In vitro, heat-inactivated, naive or sensitized baboon sera containing xenoreactive natural or elicited antibodies were used to activate porcine aortic endothelial cells (PAEC) in vitro. Tissue factor expression on PAEC was determined as an index of heightened procoagulant activity. In vivo, porcine renal xenografts were transplanted into immunosuppressed baboons, and at the time of rejection or the development of a consumptive coagulopathy, biopsy specimens were obtained for studies of xenoreactive antibody binding and tissue factor expression. Results. In vitro, incubation of PAEC with naive baboon sera containing natural anti-Galα1,3Gal (Gal) antibodies resulted in minimal tissue factor induction; the addition of complement boosted procoagulant responses. Elicited xenoreactive antibodies, and to non-Gal epitopes alone, induced high amounts of procoagulant activity on PAEC; the addition of complement resulted in overt cytotoxicity. In vivo, AVR was associated with xenoreactive antibody deposition in the graft. When vascular endothelial binding of xenoreactive antibody was combined with the expression of tissue factor, consumptive coagulopathy developed irrespective of histopathologic features of AVR. Conclusions. Our in vitro results indicate that elicited antibodies, potentially to non-Gal epitopes, induce endothelial cell activation and tissue factor expression; in vivo, a consumptive coagulopathy occurred when there was xenoreactive antibody deposition and increase of tissue factor.