The feasibility of grafting thyroid tissue as a test for histocompatibility or immunocompetence has been explored in rats. The viability of a graft was assessed by its ability to concentrate radioiodine. S.c. transplantation was successful in 90% of syngeneic or parental to F1strain combinations. Transplantation under the kidney capsule did not increase the proportion of successful grafts nor improve their functional capacity. The presence of an intact thyroid did not inhibit implantation of a grafted lobe but reduced the amount of radioiodine concentrated by it.