In the course of light and electron microscopic studies of 142 surgically-removed human pituitary adenomas, 28 tumors were found containing fibrous bodies composed of type II microfilaments with an average width of 115A. These spherical structures, measuring up to 4-5 micrometer occur exclusively in sparsely granulated growth hormone cells and acidophil stem cells, but as revealed by the immunoperoxidase technique, contain no growth hormone. Fibrous bodies are located in the Golgi region and are consistently associated with Golgi membranes and smooth-surfaced endoplasmic reticulum. Their association with centrioles is thought to be anatomical rather than functional. Several adenoma cells possess spherical formations composed entirely of smooth-walled membranes or transitional forms between smooth tubules and type II microfilaments, suggesting that smooth membranes may play a key role in the production of fibrillar substance. Fibrous bodies appear to be reliable morphologic markers and are valuable in the differential diagnosis of pituitary adenomas.