Periodontal ligament cells may have a role in the regulation of hard and soft periodontal tissues, but their specific function has yet to be determined. To evaluate further their role in periodontal homeostasis, they were examined for osteoblast-like behaviour; in vitro no characteristic osteoblastic responsiveness was found. Periodontal ligament cells gave a PGE2- and isoproterenol-mediated cAMP response, but did not respond in a similar fashion to calcitonin or PTH. When exposed to PGE2, isoproterenol, or 1,25(OH)2 vitamin D3, they did not exhibit an increase in protein production, as measured by [35S]-methionine incorporation. Immunofluorescent localization indicated that periodontal ligament cells produce a bone-associated protein, osteonectin. In addition, mRNA levels for osteonectin and bone proteoglycan I (biglycan) were detected in these cells, in vitro. This information should help to clarify the role such cells play in the regulation of periodontal tissues.