BACKGROUND: In pig-to-primate/human xenografts, hyperacute rejection of primarily vascularized organs usually occurs in 10-60 min and is due to the reaction of the recipients' natural antibodies with antigens expressed on the donor endothelium, the fixation of complement, and ultimately vascular stasis and hemorrhage. Surprisingly, the major target of the natural antibodies is the disaccharide galactose-alpha(1,3)galactose (Gal alpha(1,3)Gal), which is found on many different molecules in pig tissues and reacts with naturally occurring human anti-pig IgM and IgG antibodies. There are a number of strategies to remove/block/alter Gal alpha(1,3)Gal expression in pig tissues, all of which involve the expression of transgenes in pigs. To overcome the difficulty of preclinical studies using primates, we describe a model of hyperacute rejection of heart transplants to Gal o/o mice, which are similar to humans in that they have anti-Gal alpha(1,3)Gal antibodies. METHODS: Gal o/o mice received skin or heart grafts from Gal+ mice or rats, and additional antibody and complement were provided; hyperacute rejection was monitored by observation and histology. RESULTS: Gal alpha(1,3)Gal+ mouse tissues (skin or heart) are not rejected by Gal o/o mice. This was not unexpected, as mice do not utilize alloantibody/complement systems satisfactorily in experimental transplantation studies. However, with the addition of anti-Gal alpha(1,3)Gal antibody and complement, hyperacute rejection of hearts can occur in 10-20 min; it is mediated by IgM, not IgG, antibodies and leads predominantly to tissue hemorrhage. CONCLUSION: Gal alpha(1,3)Gal antigen modification by expression of the H transferase cDNA leads to “indefinite” survival (>120 min) and no hyperacute rejection, which shows that this model is suitable for the study of antibody-mediated rejection of relevance to pig-to-human xenografts.