Apatite formation on the surface of three kinds of bioactive material at an early stage after implantation in bone was studied using transmission electron microscopy (TEM). The materials were apatite- and wollastonite-containing glass-ceramic (A-W GC) as a surface-active glass-ceramic, dense sintered hydroxyapatite (HA) as a surface-active ceramic, and dense sintered beta-tricalcium phosphate (beta-TCP) as a resorbable ceramic. Particles of these materials, ranging from 100-300 microns in diameter, were implanted into rat tibiae, and specimens were prepared at 3, 7, 10, and 14 days after implantation. For A-W GC, dissolution of the glassy and probably wollastonite phase was observed in the surface region on and after the third day, and a collagen-free thin apatite layer on the surface of the material was evident on and after the seventh day. This apatite layer was observed before the mineralization of the surrounding bone matrix and was sometimes evident even where the material bordered on the bone marrow. On and after the tenth day, the surrounding bone matrix calcified and A-W GC-bone bonding through an apatite layer was completed. For HA, a mineralized collagen-free layer was observed on the surface of the ceramic on and after the tenth day. This layer was always present near calcifying bone and it was difficult to distinguish from immature bone. For beta-TCP, such a surface mineralized layer was rarely evident, even just before bone-ceramic contact, and finally the bone bonded to beta-TCP directly. Cell-mediated degradation of beta-TCP was frequently observed.(ABSTRACT TRUNCATED AT 250 WORDS)