Recent advances in human kidney transplantation have brought the need for a more careful analysis and understanding of histocompatibility in man. The results of human renal allografting indicate that although parents or siblings are the best donors, grafts from unrelated donors occasionally survive for prolonged periods (1), presumably due to chance compatibility. A means for measuring histocompatibility in man is, therefore, needed. To this end, a number of experimental approaches have been proposed, namely leukoagglutination (2-4) and lymphocytotoxic serotyping (5, 6) of tissue antigens; interaction of allogeneic lymphocytes in vitro (7-9); cross reactions in skin transplantation, the so-called third man test ( 10); skin reactions produced by lymphocytes in irradiated hamsters (11); and the normal lymphocyte transfer reaction (NLT). This latter phenomenon, described by Brent and Medawar (12) in the guinea pig, is a result of the transfer of viable peripheral blood lymphocytes from one individual to an intradermal site in another individual. The immunologically competent cells are presumably able to react in a graft vs. host fashion against the histocompatibility antigens in the dermis of the cell recipient, forming a reaction that is expressed as an inflammatory nodule in the skin, reaching its peak approximately 48 hours after inoculation. In the guinea pig the intensity of the reaction was shown to cor-