A prolactin-secreting pituitary adenoma was removed in a 38-year-old man. The tumor was classified as a chromophobe adenoma on the basis of tinctorial staining, but immunocytochemistry revealed that cells reacted with anti-PRL serum. Electron microscopy showed secretory granules and evidence of high metabolic activity (well developed Golgi apparatus and rough endoplasmic reticulum). Immunoelectron microscopy by the immunogold staining method allowed a clear definition of prolactin containing granules.