Suggestions that endocrine tissue is privileged, in that it may not be as severely damaged by the allograft reaction, provided the stimulus for an evaluation of thyroid transplantation across various histocompatibility barriers and into a number of different recipient sites. Results of allografting reveal that Ag-B compatibility allows for prolonged survival of thyroid tissue (much longer than skin in the same histocompatibility combinations) in the following sites: intramuscular, intradermal, renal subcapsular, and anterior chamber. Only the intrasplenic site, however, did not allow survival of even weakly incompatible grafts. In addition, all recipients of thyroid allografts rejected subsequent donor strain skin grafts in an accelerated manner, attesting to the normal antigenicity of thyroid. This finding, that thyroid is a "relatively privileged" tissue, has not been reported previously and is encouraging, since it suggests the potential success of clinical transplantation of other endocrine tissue less easily replaced than thyroid by administration of naturally occurring of synthetic analogues. Further, mild immunosuppression may serve to prolong survivals of allografts of endocrine tissue such as parathyroid or pancreatic islets, especially if an immunologically privileged site is employed.