Abstract 1. 1) Analysis of nonreactivity in FRA lymphocytotoxicity assays allows the identification of ‘acceptable mismatches’. 2. 2) A few amino acid substitutions seem to account for 75% and 90% PRA in the sera from two patients, GH and ML. 3. 3) When the ‘acceptable mismatches’ of GH and ML were aligned against the ten most common HLA-A, B haplo-types in the UK, half were judged ‘acceptable’. 4. 4) In view of the ample opportunity for sensitization to two of these antigens (HLA-B7 and -B8) through a previous kidney rejected by GH these data suggest the existence of some form of programmed nonresponsiveness to common phenotypes. 5. 5) The tendency for recipient selection schemes to siphon off kidneys with common HLA phenotypes for beneficially matched nonsensitized recipients discriminates against HSPs. 6. 6) The imagined risk of transplanting kidneys with acceptable mismatches should prove groundless if nonreactivity can be clearly shown to be attributed to energy or tolerance.