There are many diseases associated with hard tissue formation in the oral cavity. The hard tissues are derived from bone-like tissue, from cementum-like tissue and by simple calcification. It is important to distinguish the type of hard tissue to determine the classification and biological characterization of the disease. We immunostained disease specimens associated with hard tissues using a monoclonal antibody (MoAb) against human cementum using the Envision method. In normal tissue this MoAb produced a positive reaction in cementum and periodontium, and a negative reaction in bone. In cemento-osseous lesions, some regions produced a positive reaction while others were negative. The former tissue seemed to be cementum-like, while the latter seemed bone-like. These results indicate that this MoAb is useful for differentiating cemento-osseous lesions. Use of this MoAb provides new insights into the nature of these lesions.