Superparamagnetic hybrid nanoparticles (ca. 80 nm) are obtained. They consist of an inner iron oxide core coated by a silica shell, which is in turn functionalized with amine or carboxyl groups and covalently coupled to a monoclonal antibody (anti-hCG; hCG = human chorionic gonadotropin). The prepared nanoparticles show a specific magnetic moment (per gram of iron) that is comparable to that measured for commercial superparamagnetic iron oxide preparations. The bioactivity of the antibody-conjugated magnetic nanoparticles is verified by a standard bioassay. These results indicate the potential of the hybrid nanoparticles prepared for use as enhanced contrast agents in magnetic resonance imaging applications.