It was noted that many sera of patients with renal allograft produce distinct precipitation lines in gel diffusion tests with about 20% of infectious mononucleosis sera. The antibodies in infectious mononucleosis sera were of IgM isotope, but, interestingly, they could be removed by guinea pig kidney homogenate, which indicated that the reactions studied were of the Hanganutziu-Deicher rather than of the Paul-Bunnell type. This contention was strengthened by the fact that positive transplantation sera reacted also with standard serum with Hanganutziu-Deicher antibodies. Thus far, the presence of the antigen in the transplantation sera could not be related to the clinical status of the patients, however, the antigen was noted primarily in those sera that did not contain heterophile transplantation antibodies. It was proposed that the antigen detected in the transplantation sera was an altered tissue antigen released from the grafted organ. Besides, interactions between two serum samples from the same patient were noted in immunodiffusion tests. These reactions occurred very seldom and were unrelated to heterophile transplantation antigens or antibodies.