The immunohistochemical detection of elements of the human thymic extracellular matrix in situ and in vitro is described. In the normal thymus, the intracapsular and intraseptal fibers were strongly labeled by anti-type I collagen antiserum. Basement membranes bordering the capsule, septae, and perivascular spaces were intensely stained by anti-type IV collagen, anti-fibronectin, and anti-laminin sera. In hyperplastic myasthenia gravis thymuses, the major changes consisted of discontinuities of the basement membrane adjacent to clusters of epithelial (keratin-containing) cells, among which an unusual connective framework (densely labeled by all the antisera) was observed. In vitro, most epithelial cells were strongly labeled by antifibronectin serum and to a lesser extent by the anti-type IV collagen and anti-laminin sera. In addition, fibronectin, laminin, and type IV collagen were detected in the intercellular spaces bordering the epithelial cells in culture. Results show that thymic epithelial cells participate in the synthesis of extracellular matrix elements, which as a result of their localization and influence on epithelial cell growth, should be regarded as constitutive components of the thymic microenvironment.