The use of magnetic nanoparticles as contrast agents in biomedical applications requires highly reproducible markers soluble in aqueous media. Here we present a modular approach for the formation of a magnetic contrast agent, which is composed of an inorganic core like iron oxide and a water solubility mediating polymer shell. The shell can also be a carrier for specific affinity molecules like antibodies or peptides, which offer the possibility to selectively target surface receptors upon cells.