The distribution of the extracellular matrix, including type I, III, IV and VI collagens and laminin, and of prolyl hydroxylase was investigated in the human endometrium by an indirect immunofluorescence method with specific monoclonal antibodies. Collagens were also extracted from the endometrial tissues in the proliferative and secretory phases and from the decidual tissues in the first trimester of pregnancy. Immunohistochemical studies demonstrated that interstitial collagens, such as type I, III, and VI collagens, were localized diffusely in the stroma of the endometrium throughout the menstrual cycle, as well as in the decidua. Type IV collagen and laminin were localized exclusively in the basement membrane of the endometrial glands and in the walls of blood vessels during the proliferative and secretory phases. However, strong staining for type IV collagen and laminin was recognized in the pericellular region of endometrial stromal cells in the decidua. Prolyl hydroxylase was localized in the cytoplasm of endometrial stromal cells and endometrial glandular cells during the menstrual cycle. Intense immunostaining for prolyl hydroxylase was observed in the decidual cells. However, immunoreactivity for prolyl hydroxylase in the endometrial glandular cells disappeared during the process of decidualization. The ratio of type III to type I collagen was significantly decreased (P < 0.05) and the ratio of type V to type I collagen was significantly increased (P < 0.01) in the decidua, as compared with ratios in the endometrium during the proliferative phase. The present results suggest that changes in the extracellular matrix may play an important role in implantation, in invasion of trophoblastic cells and in the maintenance of pregnancy.