The interfaces between four kinds of surface-active ceramic and bone were studied by scanning electron microscopy (SEM) and transmission electron microscopy ( T E M ) using undecalcified specimens. The materials were Bioglassm-type glass (Bioglassm), Ceravital-type glassceramic (KGS), apatite-and wollastonitecontaining glass-ceramic (A-W . GC) and hydroxyapatite (HA). Particles of these materials, ranging between about 100 and 300 pm in diameter, were implanted into rat tibiae, and specimens were prepared for observation at 8 weeks after implantation. All materials were observed to bond to bone through a collagen-free layer consisting of fine apatite crytals distinct from those in bone. The crystals of this apatite layer and those of bone were intermin-gled at their interface, suggesting chemical bonding. In Bioglassm, which had only a glassy phase, several tens of microns of the material surface had changed to such an apatite layer. In KGS and A-W . GC, which had macrocrystals in the glassy phase, an intervening apatite layer about 0.5 pm thick was observed between the materials and bone. Furthermore, fine apatite crystals were also observed among the macrocrystals near the surface of the materials. In HA, which had no glassy phase, an intervening apatite layer was much less distinct and sometimes absent. These differences were considered to be attributable to the differences in chemical composition, crystallization, and solubility of the materials.