The possible role of fibronectin in dental tissue repair was investigated by comparing its distribution and that of the 165-kDa fibronectin-binding membrane protein (165 kDa-FnBP) in odontoblasts underlying carious and sound dentine. By immunoperoxidase and light microscopy, fibronectin was localized in the dentine underlying the carious lesion, mainly on the surface of the tubule walls, whereas it could not be detected in neighbouring sound zones. The antibody to the 165 kDa-FnBP strongly reacted with the membrane of odontoblasts underlying the lesion, although those facing sound dentine did not express this antigen. Ultrastructurally the 165 kDa-FnBP was localized in the cell membrane at the apical portion of odontoblasts, including the process membrane, beneath the initial lesion; fibronectin was detected in the dentinal area close to the process, and also in contact with its external surface. By a high-resolution immunogold procedure, the proteins were colocalized at the external surface of odontoblast processes. These data suggest that fibronectin present in human carious dentine could modulate the behaviour of underlying odontoblasts by means of newly expressed 165 kDa-FnBP.