The use of dynamic magnetic susceptibility measurements is reported to study nanoparticle-protein interactions in situ. The technique consists of measuring the rotational diffusivity of thermally blocked magnetic nanoparticles (MNPs) in protein solutions. To illustrate the technique, the effect of nanoparticle zeta potential in carboxymethyl-dextran-coated MNPs and their interaction with model anionic and cationic proteins, such as bovine serum albumin (BSA), immunoglobulin G (IgG), fibrinogen (FIBR), apo-transferrin (TRANS), lysozyme (LYZ), and histone (HIS), in a range of protein concentrations is studied. Experiments indicate that interactions between the negatively charged particles and the negatively charged proteins BSA, IgG, FIBR, and TRANS are negligible. However, positively charged proteins LYZ and HIS readily absorb onto the nanoparticles, as evidenced by an increase in size and eventual aggregation of the particles. Onset of this effect seems to happen at a lower concentration of HIS compared with LYZ. The technique could be applied to other particle surface coatings and to particles in complex protein mixtures, such as whole blood and serum, allowing systematic in situ studies of nanoparticle-protein interactions.