The increasing shortage of human organs for allotransplantation has created intense interest for xenotransplantation. Pig organs have been suggested as suitable grafts for man. However, the immediate problem associated with discordant xenotransplantation is hyperacute rejection. Hyperacute rejection occurs because antibodies become attached to the endothelial cells of the xenotransplant organ, which activates the complement system, attracts platelets etc. This results in endothelial cell damage, oedema, loss of vascular integrity and thus rapid destruction of the graft. Organs from non-human primates often do not cause hyperacute rejection, but will probably not be acceptable donors. Transgenic pigs have been breeded which contain the genes for the human regulators of complement activation. It appears that insertion of such genes into pigs serves to prevent aggressive hyperacute rejection. Understanding the mechanisms of rejection gives cause for optimism. However, there are still many problems to evaluate and solve. One potential danger associated with xenotransplantation is the transmission of infectious disease from animals to man. Other problems relate to physiological adaptation of the xenograft. The legal, ethical, religious and economic aspects must also be taken into consideration. Despite the potential problems, it is likely that a xenotransplant of some kind will soon be attempted in man.