Multinucleated cells (MNCs) that appeared after hydroxyapatite (HAP) implantation into experimentally-produced bone defects in rat periodontal tissues were investigated both ultrastructurally and ultracytochemically. At day 5 after implantation, MNCs first appeared along the HAP surface. They had no features of typical osteoclasts such as ruffled border and clear zone. By d 14, these cells acquired features similar to osteoclasts, including ruffled border and clear zone. With the appearance of ruffled borders in MNCs, new bone deposited around the implanted HAP. MNCs appeared to excavate both newly-formed bone and implanted HAP simultaneously. Ingested HAP particles were observed not only in MNCs but also in macrophages. MNCs contained both tartrate-resistant acid phosphatase (ACPase) and carbonic anhydrase (CAase). ACPase activity was detected along all the biosynthesizing pathways in MNCs. Extracellular ACPase activity around the ruffled border region was also demonstrable. CAase activity could be detected only in the cytosol, vesicles and mitochondrial cristae of the MNCs. These cytochemical characteristics were almost the same regardless of the time elapsed after implantation.