In 1991, evidence was provided to indicate that hyperacute rejection of pig organs by humans and baboons was initiated by the binding of anti-agalactosyl (αGal) antibodies to the vascular endothelium. A search for suitable donor species that do not express αGal epitopes has demonstrated their absence in birds, including ratites (e.g., ostrich, emu) and a reptile (alligator), and very weak expression in a large rodent (the capybara). Studies on ratites would suggest that they are unlikely to be suitable organ donors for humans. In 1991, it was also proposed that anti-αGal antibody-antigen binding could be prevented either by depletion of antibody using an immunoaffinity column of the specific αGal oligosaccharide, or by the continuous intravenous infusion of the oligosaccharide, leading to binding (and thus inactivation) of the antibody to the αGal hapten. Synthetic αGal oligosaccharides have recently become available in sufficient quantities to allow experience to be gained with immunoaffinity columns, which have demonstrated complete, but temporary, elimination of baboon serum cytotoxicity to pig PK15 cells in vivo. A search for a natural and inexpensive source of αGal has revealed a subfraction of porcine stomach mucin that demonstrates high potency in inhibiting anti-αGal antibodies both in vitro and in vivo. As an alternative therapeutic option, anti-idiotypic antibodies have been produced in mice that are specific for human and baboon anti-αGal antibodies. Their intravenous administration to baboons results in immediate reduction in serum cytotoxicity for 24 hr. Their use in delivering a toxin, such as ricin, to the B lymphocytes that produce the antibody (and express the same idiotype) is suggested. Studies on complement depletion by the administration of purified cobra venom factor (CVF) to baboons have demonstrated that, even in the absence of measurable levels of CH50, both C1q and properdin are deposited on the vascular endothelium, and that IgM, IgG, and IgA bind to the endothelium of a transplanted pig heart. The prior identification of anti-αGal IgM, IgG, and IgA by our group suggests that these antibodies might be playing a role in both classical (by IgM and IgG) and alternative (by IgA) complement pathway activation in this experimental model and in pig-to-human xenotransplantation. CVF contains a terminal αGal structure, and its administration to an otherwise unmodified baboon leads to a massive increase in anti-αGal level, suggesting that anti-CVF antibodies may, at least in part, comprise anti-αGal. The important role of anti-αGal antibodies in xenograft rejection has now been clearly established, and techniques for preventing expression of αGal epitopes in donor animals, either by genetic engineering or by gene therapy, are discussed. © Munksgaard, Copenhagen.