Bioactive organic-inorganic hybrids were synthesized through sol-gel processing starting from γ-methacryloxypropyltrimethoxysilane. NMR-spectroscopic studies showed the presence of silanol groups (Si-OH) and Si-O-Si bonds. In vitro tests of the hybrids for bioactivity with a simulated body fluid (Kokubo solution) indicated that only calcium-containing hybrids could form apatite on their surfaces. Thus the presence of calcium ions was no less important to deposit apatite than the formation of silanol groups or Si-O-Si bonds.