Bioactivity was investigated for several organically modified silicates (Ormosils) prepared through sol-gel processes. Ca(II)-free samples were biocompatible only, but Ca(II)containing samples were bioactive and deposited apatite during immersion in a simulated body fluid. The ease of silanol (Si-OH) group formation on the ormosils was considered a predominant factor controlling the bioactivity, while the effect of dissolved Ca(II) ions to increase the degree of supersaturation in the simulated body fluid is secondary.