The purpose of this article is to synthesize poly(hydroxyethyl methacrylate-N-methacryloyl-(L)-tryptophan) [mPHEMATrp] magnetic nanoparticles for lysozyme purification from chicken egg white. mPHEMATrp nanoparticles (38 nm in diameter) were synthesized by surfactant-free emulsion polymerization. Specific surface area of the mPHEMATrp nanoparticles was calculated to be 896 m(2)/g. Elemental analysis of mPHEMATrp nanoparticles was estimated in the range of 5.7-48.3 mu mol MATrp/g. The maximum lysozyme adsorption capacity of the mPHEMATrp nanoparticles was 376.1 mg/g. The mPHEMATrp nanoparticles could be used five times without decreasing the lysozyme adsorption capacity significantly. The results indicate that the mPHEMATrp nanoparticles promise high selectivity for lysozyme purification.