Abstract:" Root resorbing tissue " is a specific type of granulation tissue which is located between the roots of deciduous tooth and the developing permanent tooth germ and is responsible for the root resorption of deciduous tooth. It contains odontoclasts, macrophages, fibroblasts, neutrophils, cementoblasts, etc. Odontoclast is morphologically quite similar to osteoclast and is considered to play a major role in root resorption.It has been reported that active root resorbing tissue has a strong collagenolytic activity, but the role of collagenase in root resorption remains unclear. In order to elucidate the type of collagenase-producing cells and the role of collagenase in root resorption, in situ hybridization of bovine root resorbing tissue sections were conducted with digoxigenin-labeled non-radioactive RNA probes. Collagenase mRNA expression was clearly obserbed in odontoclasts in addition to macrophages, fibroblasts, odontoblasts and cementoblasts. Multinuclear odontoclasts showed intense TRAP activity as well as osteoclasts. Interleukin-1 (IL-1) expression in root resorbing tissue was also examined by in situ hybridization.IL-1 transcripts were found to be expressed in odontoclasts, fibroblasts and macrophages, suggesting that IL-1 might be an important factor for controlling root resorption. These results suggest that collagenase produced by odontoclasts may play a key role in dentin collagen degradation in root resorption.