Periapical granuloma is a common chronic inflammatory lesion and is considered the forerunner or initial stage of the radicular cyst. It has been well established that prostaglandins, osteoclast-activating factor and LPS are responsible for bone resorption. Few studies, however, have been done on the degradation of collagen in the bone matrix.We extracted high collagenase activities directly from human periapical grnuloma (47.8-734.6 U/g tissue, n=18) and experimental dog granuloma (52.8-286.9 U/g tissue, n=22) with 4M urea solution. Either human and dog collagenase activities were inhibited by calf serum, EDTA, o-phenanthroline, DTT, and bovine dental pulp collagenase inhibitor. SDS-PAGE of the reaction mixtures revealed that both enzymes were typical vertebrate collagenases. They broke down collagen I selectively over collagen III, which suggests the collagenase activities were mainly derived from PMN leukocytes. These results indicate that both human and dog periapical granulomas closely resemble each other in respect to the collagenase activity. Therefore, dog granuloma seems to be a good experimental model for human granuloma.