Abstract
This report describes a search for tumor-specific transplantation antigens in man by testing four sets of identical siblings using the mixed leukocyte transformation reaction. In each case, one member of the set had acute leukemia in relapse. In no instance did the leukemic cells of the patient stimulate the cells of the normal twin, nor did cells from the normal twin stimulate cells from the leukemic patient. Possible explanations for the failure to detect a leukemia-associated antigen in these studies are discussed.