A 47‐year‐old man with clinical manifestations of hyperparathyroidism was shown to have a parathyroid adenoma which in light‐microscopy chiefly consists of oxyphilic cells and transitional forms of chief cells and oxyphilic cells. Electron‐microscopy shows that the oxyphilic cells contain tightly packed mitochondria and that the transitory cells are rich in mitochondria. In these latter cells there are signs of stimulated protein synthesis. It is probable that these cells produce the excess of parathyroid hormone. Oxyphilic cells would seem to be developed from the chie