Calcium phosphate is a natural biomineral and therefore possesses an excellent biocompatibility due to its chemical similarity to human hard tissue (bone and teeth). Calcium phosphate nanoparticles can be precipitated under controlled conditions and used as carrier in biological systems, e.g. to transfer nucleic acids or drugs. Such nanoparticles can also be suitably functionalized with fluorescing dyes, polymeric agents, pro-drugs or activators. The small monodisperse nanoparticles only mildly influence the intracellular calcium level and therefore are not toxic for cells.