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Osteoadherin (OSAD) is a keratan sulfate-containing proteoglycan, belonging to the small leucine-rich proteoglycan (SLRP) family. In bone OSAD has been localized in primary spongiosa within the bovine fetal rib growth plate. Moreover, in situ hybridization has shown expression of OSAD in osteoblasts close to the cartilage and bone border in the growth plate of rat femur. mRNA expression has also detected OSAD in mature osteoblasts on the surface of bone trabeculae. We have identified OSAD in extracts of bovine dentin, and the identity was verified by N-terminal sequencing. Western blot analysis detected two bands in bovine bone and dentin at 85 kDa and 60 kDa. Northern blot analysis of RNA samples from 5-d-old-rat tooth and femur showed a 1.9-kb transcript for OSAD in both tissues. OSAD was located to the mineralized dentin matrix, cementum and surrounding alveolar bone by immunohistochemistry, and in situ hybridization showed OSAD to be highly expressed during early crown formation in the entire odontoblast cell layer, in the area of Hertwig's epithelial root sheath, in the cells of the newly formed mantle dentin, and in the odontoblasts at the fissures. Ultrastructural studies indicated that OSAD might be associated with collagen fibrils. Thus, OSAD may play an important role during tooth development and biomineralization of dentin.