In a series of scanning electron microscopic studies, the reaction of osteogenic cells to hydroxylapatite (HA) implant materials was investigated in culture and following implantation. Tissue components as found in normal rat and dog bone were identified at the interface in both test systems. In vivo, implant bed cells showed an intimate contact with the HA surfaces. Osteoblasts deposited organic matrix and bone mineral in direct apposition to HA, with no evidence of encapsulation or granulation tissue. The development of such a direct spatial relationship appears to involve mechanisms more encompassing than epitaxis.