The distribution of fibronectin throughout the basement membrane of Hertwig's epithelial sheath was studied using specific antibodies with the immunoperoxidase technique in both light and electron microscopy. Our results demonstrate that, after collagenase digestion in situ, the basement membrane was strongly labelled by antifibronectin antibodies on the lamina lucida, the lamina densa and the lamina (pars) fibroreticularis which contained aperiodic fibrils of 5-10 nm in diameter.