To investigate the bone forming activity of composite grafts of hydroxyapatite (HA) and autogenous calvarial periosteum, porous HA rods with or without the periosteum were implanted into the back muscle of white rabbits. They were harvested 3 and 6 months after implantation and studied histologically and microradiographically. Bone formation in the pores of HA occurred in three out of four composite implants of the 3 month group, and four out of five implants of the 6 month group. The two specimens without bone formation revealed irregular calcified tissue outside the HA implant. No formation of bone and calcified tissue took place in any of the specimens with HA alone. These results indicate the potential of this new experimental system of ectopic osteogenesis and offer the possibility of a new improved bone graft consisting of HA and autogenous periosteum.