Up to now little is known about cell-mediated degradation of biomaterial surfaces, especially as methodologically significant studies are not available. Therefore, the present study focused on ultrastructural details of cells involved in degradation of calcium phosphate ceramics. For the experimental procedure six adult sheep were used. At the medial aspect of the left hindlimb a cylindrical defect was created at the level of the proximal epiphysis of the tibia. Subsequently, a calcium phosphate paste was packed into the defect. Six weeks after implantation, specimens from the implants fixed by perfusion were examined histologically and by transmission electron microscopy. The results of light microscopy revealed substitution of the ceramic by newly formed lamellar bone. Electron microscopy indicated multinucleated cells localized at the implantation site and the bone surface, corresponding to osteoclasts. They formed resorption lacunae and revealed typical ultrastructural features such as the ruffled border and the sealing zone. Osteoclast-mediated degradation was performed by simultaneous resorption and phagocytosis. For the first time this degradation mechanism was documented in vivo. It confirms the notion that osteoclasts are members of the monocyte/macrophage family.