It is widely believed that the hyperacute rejection of vascularized xenografts in the pig-to-human combination is triggered by the binding of human preformed natural antibodies (PNAbs) to the Galalpha.(1,3)Gal epitope in pig endothelium and the subsequent activation of complement. However, it remains poorly defined whether xenogeneic pig pancreatic islets are damaged by antibody and complement-mediated mechanisms. We examined the expression of Galalpha(1,3)Gal on isolated adult pig islets and the presence of PNAbs in normal human sera directed against islets, using immunofluorescence staining and confocal laser scanning microscopy. The pig islets were not stained with Galalpha(1,3)Gal-specific lectin GSIB4, however, the exocrine cells reacted strongly with GSIB4, indicating that the Galalpha(1,3)Gal epitope was highly expressed on exocrine cells, but not on islets. Human sera showed weak reactivity of IgM and IgG class PNAbs to the islets, but strong reactivity to the exocrine cells. Furthermore, we investigated the cytotoxic effect of human serum on pig islets using an in vitro model of pig-to-human islet transplantation. The incubation of pig islets with normal human sera for 45 min resulted in less than 10% specific lysis despite the binding of PNAbs, whereas exposure of porcine aortic endothelial cells to the same human sera caused 56% complement-mediated lysis, determined using a MTT cytotoxic assay. These results support the view that pig islets might not undergo early antibody and complement-mediated rejection in humans.