In 1953, Billingham described a state of "actively acquired tolerance" to skin allografts developing after transfer of viable allogeneic cells to fetal or neonatal mice (1). This was followed by the identification by Mitchison of the "radiation chimaera" (2) and the demonstration by Main and Prehn of increased survival of specific donor strain skin allografts in x-irradiated and bone marrow reconstituted hosts (3). More recently, Strober (4) reported the cases of three patients treated with total lymphoid x-radiation who subsequently accepted cadaveric renal allografts. There have been no reports of acquired immunologic tolerance to specific organ allografts in humans.