Tenascin is an extracellular matrix glycoprotein, which has been reported to be involved in parenchymal-mesenchymal interactions during morphogenesis, wound healing, and carcinogenesis. Tenascin immunolocalization was performed in 51 specimens of morphologically normal human ovaries by using a specific monoclonal antibody against purified human fibroblast tenascin. In preovulatory follicles, no significant immunoreactivity was detected. In functioning corpora lutea, immunoreactivity was present as a fine border around the periphery. In association with the involution of the corpora lutea, marked diffuse tenascin immunostaining in the intercellular space was observed. These data raise the question of whether tenascin may be involved in luteolysis and may play an important role in the ovarian cycle by regulating the involution of corpora lutea.