Antibody-functionalized magnetic nanoparticles dispersed in phosphate-buffered saline solution were used as reagents in immunomagnetic reduction assays. Biomolecules are detected in bioliquid samples when they associate with magnetic nanoparticles and reduce the AC magnetic susceptibility χac of the reagent at a given frequency. In this study, the chemical kinetics for the real-time χac during the association was investigated. The association kinetics between biomolecules and nanoparticles consists of diffusion and binding steps. It was found that the diffusion speeds up in samples with higher concentrations of molecules. Furthermore, the period of association was longer for samples having higher concentrations of molecules. The association rates were proportional to the T-Tau concentration. The results showed that one biomolecule was associated with one magnetic nanoparticle.