Ten cases of Medullary carcinoma of Thyroid gland (MCT) were examined (two familial, eight sporadic) in patients with increased serum calcitonin levels and ranging age from 31 to 87 years. Immunocytochemical reactions were performed using Peroxidase-Antiperoxidase (PAP) complex and/or Avidin-Biotin-Peroxidase complex (ABC) by light microscopy on fixed tumorous specimens (10 cases) and additionally by electron microscopy (3 cases). The tumors displayed light microscopic features typical of MCT. The cells were polyhedral, angular or fusiform and showed high affinity for Grimelius stain. Large deposits of amyloid were Congo red stained and disclosed a green birefringence in polarized light. In the familial cases sections of non-tumorous thyroid tissue showed C cell hyperplasia. Electron microscopic studies revealed intracytoplasmic secretory granules and amyloid fibrils. The immunoperoxidase technique demonstrated calcitonin in the tumor cells in all the cases, with a weaker staining than in para-and intra-follicular C cells of adjacent non tumorous tissue and than in C cells hyperplasia foci, when present. Immunoreactive tumor cells were also observed with anti SRIF in 4 cases, with anti-ACTH and anti-beta MSH in 3 cases, and with anti-beta endorphin in 2 cases. These results are discussed with a review of the literature.