The cyclic changes in the presence of the basement membrane glycoprotein laminin in endometrial stromal cells was studied by immunohistochemistry. The interstitial matrix around the stromal cells of the proliferative phase of the normal menstrual cycle was unreactive with antibodies to laminin. However, commencing with the secretory phase, stromal cells accumulated distinct cytoplasmic and pericellular laminin-immunoreactive material. The maximal amount of stromal cell-associated laminin was observed in predecidual cells of the late secretory phase. Thus, laminin immunostaining discriminates stromal cells of the proliferative phase (being "negative") from those in the secretory phase (being "positive"). Sixty-six cases of endometrial hyperplasia and adenocarcinomas were also stained with antibodies to laminin. Sixty-nine percent of biopsies of cystic hyperplasia and 30% of adenomatous hyperplasia contained laminin-positive stromal cells. Ultrastructural examination of stromal cells in cystic hyperplasia revealed the presence of pericellular basement membrane-like material, focally arranged into typical lamina rara and lamina densa. In contrast, stromal cells in the atypical adenomatous hyperplasia and adenocarcinomas did not react with antibody to laminin. The expression of laminin receptor in the stromal cells codistributed with laminin. Basement membranes of the surface epithelium, the glandular epithelium, and the vessels stained strongly with antibodies to laminin. In preneoplastic and neoplastic tissues, laminin immunostaining revealed discontinuous and defective basement membranes. In poorly differentiated carcinomas only sparse amounts of laminin-positive basement membrane were observed; these tumors, in contrast, exhibited cytoplasmic laminin and also significant immunoreaction with antibodies to laminin receptor.