The xenotransplantation of pig organs to humans is now receiving serious consideration because of the shortage of human donors for organ transplants. However, such xenografts would be hyperacutely rejected due to naturally occurring antibodies, present in all human sera, that react with pig antigens on the surface of endothelial cells, leading to complement fixation and the rapid onset of intravascular coagulation. A major target of these human natural antibodies is the terminal nonreducing disaccharide Gal alpha (1,3)Gal, and we now report on the array of molecules that are galactosylated by the alpha 1,3-galactosyltransferase. Pig lymphocytes and endothelial cells (both of which bear Gal alpha(1,3)Gal epitopes) were surface iodinated and the 125I-labeled molecules were precipitated with either human antibodies or the lectin from Griffonia simplicifolia (IB4, which binds to Gal alpha(1,3)Gal epitopes). The precipitated molecules were analyzed by gel electrophoresis and autoradiography. Five major groups of molecules were identified by one-dimensional SDS/PAGE (alpha 220 kDa, beta 160-180 kDa, gamma 120 kDa, delta 64 kDa, epsilon 40 kDa); the beta molecule was different in the 2 cell types (beta 1 of lymphocytes and beta 2 of endothelial cells). Two-dimensional SDS/PAGE analysis revealed that each of these groups of molecules resolved into further species of different charge (presumably due to different glycosylation) and also different molecular mass to give at least 20 different Gal alpha(1,3)Gal+ surface molecules. None of these molecules appeared to be present as disulfide-associated dimers. It is clear that there are many galactosylated molecules on the cell surface; indeed, using longer exposures of the autoradiographs, at least 40 different Gal alpha (1,3)Gal+ molecules could be identified. Several of these molecules are likely to have been identified by others, e.g., the 115-kDa, 125-kDa, and 135-kDa triad identified by Platt. Strategies to overcome hyperacute rejection could include modification or deletion of the alpha 1,3-galactosyltransferase gene, which would simultaneously delete all the Gal alpha(1,3)Gal epitopes on these molecules.