Using an immunocytochemical technique, the extracellular matrix components fibronectin, vimentin, laminin and collagen type IV were investigated in human chorionic villi of various stages of development. Fibronectin and laminin were consistently positive throughout embryonic development. Vimentin and collagen type IV were negative in first and second trimester chorionic villi, but became positive in term placentas. With the exception of laminin, all extracellular matrix molecules were detected in the villous stroma and, with the exception of vimentin, they were localized in the basement membranes. Our data suggest that fibronectin and laminin are essential components of the villous structure, while the presence of vimentin and collagen type IV in the chorionic villi should be regarded as an indicator of fetal maturation.