After porcine skin allografts there is a regular rejection within 6 - 12 days with a characteristic pattern of humoral antibodies corresponding to the histocompatibility testing. The rejection of skin allografts may be due to weak histocompatibility antigens which are not measured by the SL-A and MLC typing. A porcine liver allograft induces an immunological reaction of moderate degree. This state of hyporeactivity i. e. partial tolerance could be induced and maintained by the release of tolerogenic antigens from the porcine liver.