Periodontal ligament (PDL) cells derived from mandibular first premolars, bone cells derived from mandibular cortical bone fragments and gingival cells derived from the maxillary tuberosity region were obtained from a patient during the course of surgical orthodontic treatment. Cell proliferation, cellular alkaline phosphatase (ALPase) activity, prostaglandin E2 (PGE₂) production and bone resorption activity among these three cell types were examined at the same passage in vitro. All cell types grew significantly in a time dependent manner. PDL cells were more potent than the other cell types in proliferation. The ALPase activity increased in a time dependent manner, and the level of ALPase activity from these cells ranked in the following order; bone>PDL>gingiva. The effect of cell passage with these cells for proliferation, PGE₂ and bone resorption activity was also investigated and the findings indicated that younger generation (passages 4th to 5th) of any cell type has more potent activity for proliferation, PGE₂ production and bone resorption activity than older generation (passages 6th to 7th). Moreover, the addition of indomethacin completely inhibited the PGE₂ produced from PDL cells, bone cells and gingival cells. However, pretreatment of PDL cells with indomethacin partially inhibited bone resorption activity caused by untreated PDL cells indicating that PGE₂ is not the only factor present in PDL conditioned medium that is responsible for stimulating bone resorption.