We report that multifunctional polymer nanoparticles approximately the size of a large protein can be "purified", on the basis of peptide affinity just as antibodies, using an affinity chromatography strategy. The selection process takes advantage of the thermoresponsiveness of the nanoparticles allowing "catch and release" of the target peptide by adjusting the temperature. Purified particles show much stronger affinity (K-dapp approximate to nM) and a narrower affinity distribution than the average of particles before purification (K-dapp > mu M) at room temperature but can release the peptide just by changing the temperature. We anticipate this affinity selection will be general and become an integral step for the preparation of "plastic antibodies" with near-homogeneous and tailored affinity for target biomacromolecules.