Transplantation of porcine tissue into humans induces a vigorous immune response directed predominantly at the carbohydrate epitope galactose-&agr;1,3-galactose (Gal). Gal knockout mice, like humans, do not express the enzyme responsible for synthesis of Gal and consequently produce anti-Gal antibodies in response to environmental antigens or deliberate sensitization. The Gal knockout mouse has been widely used as a surrogate recipient of Gal-expressing grafts. This small animal model has been invaluable in analysis of the immune response to Gal and in the development and testing of various strategies to prevent Gal-mediated xenograft rejection.