Physiological mediators of bone resorption stimulate osteoclasts indirectly via osteoblasts. The aim was to determine whether osteoblasts are necessary for lipopolysaccharide (LPS) to stimulate osteoclastic resorption. Osteoblasts were prepared from neonatal rat calvaria by sequential digestion, and co-cultured on slices of sperm-whale dentine with osteoclasts disaggregated from neonatal rat long bones. Groups of 12 slices were cultured for 24 h in culture media alone, or media containing either 100 ng/ml parathyroid hormone (PTH) or 0.1, 1 or 10 micrograms/ml LPS extracted from Bacteroides gingivalis. After incubation, the slices were cleaned and viewed by scanning electron-microscopy. PTH and each concentration of LPS caused a statistically-significant increase in the mean area of resorption per dentine slice when compared with controls. When only isolated osteoclasts were cultured on dentine slices and incubated under similar conditions, none of the test media significantly stimulated bone resorption above that of controls. Thus LPS stimulates osteoclastic bone resorption indirectly via osteoblasts.