Some bone-substitute biomaterials have been classified as bioactive since they allow direct biological bonding to their surface in vivo. Using in vitro techniques, we have re-created the first stages of this biological bonding phenomenon and compared the initial, fibrillar, extracellular matrices produced by migrated primary osteoblast cell populations in contact with both dense and macroporous calcium phosphate substrata, apatite/bioactive glass composite (ABC) and 45S5 bioactive glass (BAG). The first formed fibrils in contact with these materials may be identified as collagen from their morphology as observed by scanning electron microscopy (SEM). However, the organization of this fibrillar material is significantly different on the five bioactive substrata examined. These in vitro findings may not only be related to both the surface morphology and surface chemistry of the substrata, but also correlated with their levels of in vivo bioactivity.