Calcified concretions are a normal and constant finding in the anterior pituitary gland of fetuses and newborns. Their light and electron microscopic characteristics have been recently reported by the authors. In this study, undecalcified and decalcified sections from 20 neonatal and 60 fetal anterior pituitary glands were studied by histochemical and immunohistochemical methods to further clarify their nature and mechanism of formation. All the glands revealed homogeneous and/or laminar calcifications located either within the interstitium or follicular structures. They were composed of a diastase-resistant periodic acid-Schiff-positive carbohydrate-rich matrix. The Feulgen method for DNA was negative. Their core frequently reacted to Alcian blue and epithelial membrane antigen (EMA). EMA also stained the apical membranes of adjacent epithelial cells. Other immunostains (vimentin, keratin, and pituitary hormones) were negative. The positive staining for Alcian blue and EMA and the negative staining with the Feulgen method for DNA suggest that the core of the calcifications consists of acidic mucosubstances and EMA-positive proteinaceous material previously secreted by viable pituitary cells. The EMA-negative periphery of the concretions probably develops from further extracellular peripheral mineralization that leads to larger, sometimes laminated psammoma bodies. The occurrence of pituitary calcifications in states of adult physiological and pathological hyperprolactinemia suggests that the marked proliferation of lactotrophs occurring during the fetal life play an important role in the pathogenesis of the fetal and neonatal concretions.