Cathepsin K is a cysteine proteinase, which is abundantly and selectively expressed in osteoclasts. It is believed to play an important role in the proteolysis of bone resorption by osteoclasts. The objectives of this study were to investigate the association of cathepsin K in the physiological root resorption of deciduous teeth and to identify the cathepsin K-producing cells in deciduous root resorption. RT-PCR and Northern blot analysis of the total RNAs extracted from bovine active and resting root-resorbing tissues, which lie between the root of deciduous tooth and its permanent successor, were performed. The active root-resorbing tissue, which has a high population of odontoclasts on its surface that is attached to resorbing root surface, showed an extremely high expression of cathepsin K in comparison with the resting root-resorbing tissue. By in situ hybridization, cathepsin K mRNA was highly and selectively expressed in multinucleated odontoclasts that aligned along the surface of the tissue and apposed to the resorbing root surface of the deciduous tooth. Western blot analysis of the active root-resorbing tissue was used to characterize the anti-cathepsin K antibody. A band of 27 kDa, corresponding with the predicted size for mature cathepsin K, was demonstrated. Immunohistochemistry confirmed the specific localization of cathepsin K protein to the odontoclasts. These results demonstrate that odontoclasts in the deciduous root resorption express cathepsin K mRNA and protein that may participate in the proteolysis of root resorption of the deciduous tooth.