Using co-cultures of mouse osteoblastic cells and hemopoietic cells, we have shown that osteoblastic stromal cells are prerequisite to osteoclast development through cell-cell interaction. In the present study, we developed a method for obtaining functionally active purified osteoclasts and examined whether the purified osteoclasts are capable of forming resorption pits on dentine slices without the help of osteoblastic cells.Mouse primary osteoblastic cells and bone marrow cells were co-cultured for 7 days on collagen gel-coated dishes in the presence of 1α,25(OH) 2 D 3 . The co-cultures were then treated with pronase followed by collagenase to recover cell suspension containing osteoclasts. The cell suspension was layered onto 30% Percoll and centrifuged at 1,000 rpm for 30 min. Osteoclasts accumulating on the Percoll gradient were collected and used for pit formation and actin ring formation assays.The purified osteoclasts (∼70% of the purity) cultured for 24 hr on dentine slices could not form resorption pits on the slices. When various numbers of primary osteoblastic cells were added to the pit formation assay, the area of resorption pits increased strikingly with the increase in the number of osteoblastic cells added. The osteoblastic MC3T3-E1 cells could be substituted for primary osteoblastic cells to induce pit forming activity of osteoclasts, but fibroblastic C3H10T1/2 cells and NIH3T3 cells could not. When cell-cell contact between MC3T3-E1 cells and purified osteoclasts was prevented, osteoclasts could not form resorption pits on the slices. Actin ring formation in osteoclasts placed on dentine slices was well correlated with their pit-forming activity. These results indicate that osteoblastic cells are prerequisite for inducing not only osteoclast development but also for exerting osteoclast function. Very recently, Wesolowskiet al. have attained a similar conclusion.