We report the fabrication and characterization of antifouling polymer-coated magnetic nanoparticles as nanoprobes for magnetic resonance ( MR) contrast agents. Magnetite superparamagnetic iron oxide nanoparticles ( SPION) were coated with the protein- or cell-resistant polymer, poly( TMSMA-r-PEGMA), to generate stable, protein- resistant MR probes. Coated magnetic nanoparticles synthesized using two different preparation methods ( in situ and stepwise, respectively) were both well dispersed in PBS buffer at a variety of pH conditions ( pH 1-10). In addition, dynamic light scattering data revealed that their sizes were not altered even after 24 h of incubation in 10% serum containing cell culture medium, indicative of a lack of protein adsorption on their surfaces. When the antibiofouling polymer-coated SPION were incubated with macrophage cells, uptake was significantly lower in comparison to that of the popular contrast agent, Feridex I. V., suggesting that the polymer-coated SPION can be long-circulated in plasma by escaping from uptake by the reticular endothelial system ( RES) such as macrophages. Indeed, when the coated SPION were administered to tumor xenograft mice by intravenous injection, the tumor could be detected in T2-weighted MR images within 1 h as a result of the accumulation of the nanomagnets within the tumor site. Although the poly( TMSMA-r-PEGMA)-coated SPION do not have any targeting ligands on their surface, they are potentially useful for cancer diagnosis in vivo.