Terminal alpha (1,3) galactosyl galactoside epitopes (alpha-gal) on membrane glycoproteins expressed by vascular endothelial cells represent the major xenoreactive antigens in pig to primate xenotransplantation. In other discordant xenotransplantation combinations, such as from guinea pig to rat, carbohydrate epitopes other than alpha-gal may be targeted by xenoreactive antibodies (XNA). We have shown that agonist binding to alpha-gal epitopes induces proinflammatory activation of porcine aortic endothelial cells (PAEC). Binding of alpha-gal epitopes by Bandeiraea simplicifolia isolectin B4 results in both type I and type II PAEC activation. This includes the phosphorylation of tyrosine residue(s) of a protein with an apparent molecular weight of 130 kDa (p130). In order to investigate whether binding of other carbohydrate epitopes could induce a similar phosphorylation event, several lectins with different carbohydrate specificities were used to stimulate PAEC and human umbilical endothelial cells (HUVEC). In addition to BS-IB4 binding to alpha-gal, lectins binding to sialic acid isolated from Sambucus nigra (SNA), Maackia amurensis (MAA), Wheat germ agglutinin (WGA), and lectin from jack bean (Concanavalin A, ConA), that binds to mannose residues within the core structure of N-glycosylated proteins all induced the phosphorylation of the p130 protein(s). Lectins with affinity to alpha bound N-acetylgalactosamine, Dolichos biflorus (DOB), and Sophora japonoca (SOJ) did not induce this phosphorylation event. A similar negative result was obtained with Ulex europaeus lectin I, which binds to fucose residues. Conclusively, endothelial cell activation can be observed upon binding of various lectins to the glycosylated moiety of surface glycoproteins. These carbohydrate epitopes against which XNA may exist in certain models might represent minor xenoantigens from porcine to primates or may comprise the major xenoepitopes in other discordant xenograft models. Binding of XNA and subsequently the elicited xenoreactive antibodies to carbohydrate epitopes may therefore contribute to xenograft rejection even in the absence of complement inactivation.