Using extracted human deciduous teeth undergoing physiologic root resorption, this author studied the ultrastructural and cytochemical features of odontoclasts. The scanning electron microscopic observation of trypsin-treated dentin and cementum surfaces of resorption lacunae showed the exposure of collagen fibrils and prominent loss of the peritubular matrices around the dentinal tubules. In the resorption lacunae formed in enamel, there was dissolution of either the rod or the interrod regions. The odontoclasts developed extensive ruffled borders apposed to these resorbing matrices and had round phagosomes containing tannic acid-stainable fine amorphous inclusions, which were identical to those in the extracellular canals of the ruffled borders. The odontoclasts did not phagocytose the collagen fibrils. The odontoclasts showed the enzymatic activities of the acid trimetaphosphatase and acid p-nitrophenyl phosphatase (p-NPPase) in the Golgi-lysosome system, the ruffled border region, and along the resorbing dentin surfaces. The p-NPPase activity was inhibited by sodium tartrate. Also, the odontoclasts showed H(+)-K(+)-ATPase activity in the cytoplasm along the plasma membranes including those of ruffled border and the limiting membranes of the lysosomes. These results suggest that: 1) the odontoclasts are associated with resorption of non-collagenous organic matrices and/or extracellularly-degraded collagenous fragments rather than the incorporation of intact collagen fibrils; 2) the odontoclasts release the hydrolytic enzymes onto the lacunal surfaces and/or the lysosomes for the extra/intracellular degradation of the organic matrices; and 3) they also have H(+)-K(+)-ATPase for extracellular demineralization of the inorganic crystals.