Thirty-eight human pituitary adenomas (24 endocrine active and 14 endocrine inactive tumors) were studied immunohistochemically for the presence of the basement membrane component, laminin, and ultrastructurally for the presence of basement membrane. Immunoreactivity of laminin delineated staining of epithelial and endothelial basement membranes, the reaction product being confined mostly to the perivascular zones. Moreover, a hitherto undescribed presence of intercellular laminin-positive droplets was observed in ten of the active adenomas (nine patients with hyperprolactinemia and/or acromegalia and one patient with Cushing's syndrome). Concurrently, at the ultrastructural level, bunches of basement membrane-like material intermingled between the adenoma cells were demonstrated in seven of these ten active adenomas. Furthermore, secretory granules were entrapped occasionally in this intercellular matrix, indicating a mutual dependence between excessive hormone extrusion and an increase of "misplaced" deposits of basement membrane components, e.g., laminin.