Monoclonal antibodies are important therapeutic proteins. One of the challenges facing large-scale production of monoclonal antibodies is separation efficiency - a way to increase this separation efficiency is by using magnetic stimuli-responsive polymer nanoparticles. In this work thermo-responsive magnetic particles composed of a magnetic poly(methyl methacrylate) core with a poly(N-isopropylacrylamide-co-acrylic acid) shell cross-linked with N, N'-methylenebisacrylamide were prepared by miniemulsion polymerization. The particles were shown to have an average hydrodynamic diameter of 317 nm at 18°C, which decreased to 277 nm at 41°C due to the collapse of the thermo-responsive shell. The particles were superparamagnetic in behavior and exhibited a saturation magnetization of 12.6 emu/g. Subsequently, we have evaluated the potential of these negatively charged stimuli-responsive magnetic particles in the purification of a monoclonal antibody from a dialyzed CHO cell culture supernatant by cation exchange. The adsorption step was highly selective and allowed for the recovery of approximately 94% of the mAb. Different elution strategies were employed providing highly pure fractions with host cell protein removals greater than 98%. By exploring the stimuli-responsive proprieties of the particles, shorter magnetic separation times were possible without significant differences in product yield and purity.