Small amyloid deposits occur commonly in different organs in association with aging. As in other amyloids, the fibrils in the age-associated forms are built up by specific proteins, unique to every histological type. The amyloid proteins that have been identified in localized amyloid of human endocrine organs have all been of polypeptide hormone nature, and these include calcitonin, islet amyloid polypeptide (amylin), and atrial natriuretic factor. In the present study, we add prolactin to the increasing group of known amyloid proteins and show that this hormone constitutes the amyloid fibrils of pituitary glands of aging individuals.