Rejection of pig-to-human or pig-to-primate xenografts is mediated by the natural anti-Gal antibody, which interacts with α-galactosyl epitopes (ie, Galα1-3Galβ1-4GlcNAc-R) abundantly expressed on porcine cells. The objective of this study was to determine the ability of various synthetic oligosaccharides to inhibit the binding of anti-Gal IgG molecules to porcine endothelial cells in vitro. Such inhibition ultimately may help to reduce or to prevent the in vivo antibody-dependent cell cytotoxicity (ADCC) reaction. In the absence of complement-mediated hyperacute rejection, the ADCC induced by anti-Gal IgG molecules is likely to cause the chronic rejection of xenografts. The synthetic free α-galactosyl epitope (Galα1-3Galβ1-4GlcNAc) was found to be 300-fold more effective than melibiose or α-methyl galactoside in inhibiting anti-Gal binding to porcine endothelial cells, and to prevent> 90% of the antibody binding at a concentration of 1 mM. The disaccharide Galα1-3Gal was ten-fold less effective than the free α-galactosyl epitope. Accordingly, the affinity of the disaccharide to anti-Gal, as measured by equilibrium dialysis, was seven-fold lower than that of the trisaccharide. The effective concentration of oligosaccharides inhibiting anti-Gal is independent of the antibody affinity, but is dependent on the concentration of the antibody. Based on the small difference in affinity between Galα1-3Galβ1-4GlcNAc and Galα1-3Galβ1-4Glc, and the large difference in the price of N-acetyllactosamine vs. lactose, it is suggested that lactose may be considered as an appropriate starting material for synthesizing large amounts of a trisaccharide that effectively neutralizes anti-Gal.