The initiating factor in the hyperacute rejection of pig organs by human or non-human primates is believed to be related to the presence of preformed" natural" antibodies in the host. In 1991, we demonstrated that human anti-pig antibodies were IgG, IgM and IgA and bound most strongly to oligosaccharides with an alpha galactose (alpha Gal) terminal residue. These included (i) alpha Gal-R (alpha galactose),(ii) alpha Gall-3 beta Gal-R (B disaccharide),(iii) alpha Gall-3 beta Gall-4 beta GlcNAc-R (linear B type 2 trisaccharide) and (iv) alpha Gall-3 beta Gall-4 beta Glc-R (linear B type 6 trisaccharide) where R is (CH2) 8COOCH3. In vitro studies using both the chromium release assay and a live/dead staining technique demonstrated that the cytotoxicity of human sera towards pig cells can be significantly reduced or abolished by immunoadsorption of the serum with immunoaffinity columns of an alpha Gal structure, particularly those with an alpha 1-3 linkage, and not by a large selection of other carbohydrates. Similarly, human anti-pig antibodies can be largely inhibited or" neutralized" by the addition of an alpha 1-3Gal di-or trisaccharide to the serum. Staining of pig vascular endothelium utilizing a panel of carbohydrate-specific lectins and immunoaffinity antibodies demonstrated the presence of three different carbohydrate epitopes, namely (i) alpha Gall-3 beta Gall-4 beta GlcNAc-R (linear B type 2 trisaccharide (ii) alpha NeuAc2-3 beta Gall-4 beta GlcNAc-R (sialyl-N-acetyllactosamine), and (iii) beta Gall-4 beta GlcNAc-R (N-acetyllactosamine). We have investigated organs from several breeds of pig and have concluded that the alpha Gal epitope is either monomorphic or at least has a high incidence in porcine species, since we have not found any pig negative for this antigen. Human vascular endothelial cells have at their surface the same lactosamine-ended precursor and sialylated chains as pigs, but instead of terminal alpha Gal they express the fucosylated polymorphic ABH histo-blood group epitopes. As we have found no evidence that human or baboon plasma contain antibodies directed against sialic acid or lactosamine, and as human tissues contain both of these carbohydrates, it seems unlikely that either of these epitopes plays a role in the vascular rejection that takes place when pig organs are transplanted into primates. Unfortunately, the alpha Gal disaccharide and trisaccharides were not available to us in the large quantities required for extracorporeal immunoadsorption or continuous intravenous infusion in adult baboons.(ABSTRACT TRUNCATED AT 400 WORDS)