A well-developed 23-year-old man, complaining of blurred vision but with no endocrine symptoms, was found to have a large pituitary adenoma spreading outside the sella. Endocrine investigations disclosed growth hormone deficiency, hyperprolactinemia (responsive to thyrotropin-releasing hormone), and very high blood alpha-subunit (72 ng/ml) level. Histology showed a chromophobic, slightly acidophilic pituitary adenoma with focal fibrosis and calcification. The immunoperoxidase technique revealed prolactin and alpha-subunit in the cytoplasm of a single-cell type, at the light and electron microscopic level, indicating that monomorphous, plurihormonal adenomas exist in the human pituitary. Immunostaining with antibodies raised against beta-thyroid stimulating hormone, beta-follicle stimulating hormone and alpha-endorphin were observed in scattered cells. Those cells that contained immunoreactive alpha-endorphin did not appear to contain alpha-subunit. The ultrastructural features of adenoma cells showed no resemblance to any known cells in nontumorous or tumorous pituitaries. It can be postulated that adenohypophysial cells, after neoplastic transformation, may have the ability to secrete a number of biochemically unrelated hormones, suggesting that during embryologic development they pass through a common progenitor cell stage, capable of plurihormonal activity.