A single injection of 1-hydroxyethylidene-1, 1-bisphosphonate inhibits the formation of acellular cementum in rat molars. Instead, an atypical hyperplastic cementum is formed. In this study the resistance of this cementum to resorption was tested by applying an orthodontic force. It was found that roots lacking acellular cementum were readily resorbed. The readiness with which the atypical hyperplastic cementum was resorbed without any increase in multinucleated osteoblasts may offer useful opportunities to study the different phases of hard-tissue resorption.