The implantation of animal organs is one approach to overcoming the shortage of human donor organs for medical transplantation. Although readily available, non-primate tissues are subject to hyperacute rejection wherein human anti-Galalpha(1-3)Gal antibodies react with haptens present on the transplanted cells' surfaces. The understanding of this interaction on a molecular level will further the development of a strategy for the prevention of hyperacute rejection in xenotransplantation. The Galalpha(1-3)Gal hapten ('xenograft antigen') has been cocrystallized with the Gal-specific B(4) isolectin of Griffonia simplicifolia lectin-1. Crystals were analyzed by cryocrystallography and were found to diffract to moderately high resolution on a rotating-anode X-ray source. They belong to the P2(1)2(1)2 space group, with unit-cell parameters a = 111.0, b = 51.3, c = 76.9 A, and contain two molecules per asymmetric unit.