In xenotransplantation, adhesion of pig-to-human cells may play an important role in the rejection of porcine xenograft issues. Human decay accelerating factor (DAF or CD55) is an important regulator of the complement activation. Transgenic pigs expressing human DAF are being bred in the expectation that organs from these pigs will be better protected from hyperacute rejection, a consequence of the activation of complement. Recently, DAF have been identified as a high affinity ligand for the CD97 molecule, which is expressed upon activation of many !eucocytes. Hence, expression of DAF in pig organs, although abrogating hyperacute rejection, might theoretically predispose to develop.