A lthough 1,25-dihydroxyvitamin D3 stimulates osteoclastic bone resorption in vivo and in organ culture, the mechanism by which it effects this stimulation is unknown. We have recently found that the agent does not stimulate resorption by osteoclasts mechanically disaggregated from bone and incubated on slices of cortical bone. This suggests that the osteoclasts were removed by disaggregation from the influence of some cell type, present in intact bone, that mediates hormone responsiveness. We therefore tested the ability of osteoblastic cells derived from neonatal rat calvariae and of cloned, hormone-responsive osteosarcoma cells (UMR106) to restore hormone responsiveness to unresponsive populations of osteoclasts. We found that osteoblastic cells from both sources induced a two- to fourfold stimulation of osteoclastic bone resorption in the presence of 1,25-dihydroxyvitamin D3. Stimulation was observed at concentrations of 10(-10) M and above. Actinomycin D and cycloheximide did not affect bone resorption by osteoclasts incubated alone, but abolished the capacity of osteoblastic cells to stimulate osteoclastic resorption in the presence of 1,25-dihydroxyvitamin D3. When calvarial cells or osteoblastlike UMR cells were incubated with the hormone, they produced a factor in cell-free supernatants that stimulated bone resorption by disaggregated osteoclasts. These experiments suggest that 1,25-dihydroxyvitamin D3 stimulates bone resorption through a primary action on osteoblastic cells, that are induced by the hormone to produce a factor that stimulates osteoclastic bone resorption.