Monoclonal antibody against bovine serum alpha 2HS glycoprotein was raised in mice and the localization of this glycoprotein in bovine and human teeth was examined by immunohistochemical techniques on undemineralized ground sections after surface etching with dilute HCl. Positive reactions were found in both bovine and human peritubular dentin, whereas virtually negative staining was detected in the intertubular dentin. When the ground section was examined without surface etching, no reaction was found in any area of mineralized dentin. These results suggest that alpha 2HS glycoprotein is present mainly in the peritubular dentin associated with the mineral phase. The presence of this component in dentin may not be due simply to nonspecific binding to calcium phosphate crystals.