The presence of Gal&agr;1,3Gal epitopes on the vascular endothelium of pigs and of natural anti-Gal antibodies only in humans and Old World nonhuman primates necessitates the use of these nonhuman primates in xenotransplantation models involving pig organs. Recent work has concentrated attention largely on methods of maintaining low natural anti-Gal antibody levels, of preventing the production of elicited antipig antibodies, and/or of inhibiting the activity of complement. The immunologic problems related to pig lung transplantation in nonhuman primates would appear to be accelerated and to have a greater severity than those related to other pig organs. Efforts continue to induce tolerance to pig antigens by mixed hematopoietic chimerism, although this approach has been hindered by the development of a thrombotic microangiopathy directly associated with the presence of pig cells in primate blood.