The ultrastructure of "non-pathologic" human pituitary gland obtained from eight cases at autopsy, who received no hormonal therapy and who revealed no significant changes in the pituitary and its target organs, was investigated in an attempt to identify human adenohypophyseal cells by comparing their secretory granules with those of experimental animals. Besides the follicular cell, five different granulated cell types were distinguished The Type 1 cell contained abundant, dense secretory granules (350-500 mmu across). The Type II cell was characterized by dense granules, which were the largest in size (500-700 mmu across) and the most irregular in shape. Granules of the Type III cell were less characteristic (200-300 mmu across). The Type IV cell contained dense granules, which were the smallest in size (100-150 mmu across) and were characteristically arranged along the plasma membrane. The Type V cell was characterized by the presence of small, haloed granules (100-200 mmu across). The cells of these five types could presumably be the somatotroph, lactotroph, gonadotroph, thyrotroph and corcitotroph, respectively.