Cultured, viable and functioning ABO compatible parathyroid cells were allografted in 18 nonimmunosuppressed patients with a postoperative hypoparathyroidism. Variable, but promising biochemical and clinical results were obtained. Clinical and biochemical observations have documented graft function up to 14 months. The mechanism of cessation of function of implanted cultured cell suspension remains unknown. Some parameters, suggest a rejection mechanism but other mechanisms can not be excluded. This suggests that some form of immune modulation may be necessary to improve further PT allograft survival in recipients off immunosuppressive therapy.