Hyperacute rejection is a major obstacle to successful transplantation of vascularized xenogeneic organs and is believed to be mediated at least in part by performed xenoreactive "natural antibodies" (NAb). In this study, human NAb that could be involved in hyperacute rejection of pig heart xenografts were identified and characterized using an ex vivo model in which pig hearts were perfused with whole blood from individual human or pig donors. This ex vivo perfusion model allows for the continuous monitoring of physiologic parameters of cardiac function as well as sequential sampling of tissue and blood. Pig hearts perfused with allogeneic pig blood maintained normal function for at least 4 hr, whereas those perfused with xenogeneic AB+ human blood never achieved normal function and rejected completely after 30 min. In three separate experiments involving different human blood donors and pig hearts, sequential samples of perfused blood revealed a progressive depletion of anti-porcine NAb. Samples of all three rejected cardiac xenografts were homogenized, and the specifically bound human anti-porcine antibodies were eluted with citric acid. The eluted antibodies were enriched approximately 50-120-fold for anti-porcine reactivity compared with serum from the corresponding donor. Eluates contained NAb of predominantly IgM and IgG2 isotypes. Immunofluorescence histology confirmed the deposition of IgM and IgG2 but not other IgG subclasses in the rejected pig hearts. Since IgG2 utilized predominantly in response to bacterial polysaccharide antigens, our findings are consistent with the possibility that some NAb arise via crossreactivity with microbial antigens and are predominantly directed against carbohydrate rather than protein antigens.