We explore the use of magnetic relaxation sensors as an in vitro assay platform. Functionalized magnetic nanoparticles that exhibit transverse relaxation time (T(2)) changes with the introduction of an analyte have previously been used to detect proteins, antibodies, receptor ligands, peptides, nucleic acids, oligonucleotides, and other small molecules. The standard procedure for sensitizing particles towards specific targets is, however, time-consuming and cumbersome. We report here a new approach that exploits primary-secondary antibody binding for easily derivatizing particles against specific targets. The assay is shown to be quantitative and a multiplexed assay against three target analytes is demonstrated.