A novel magnetic nano-adsorbent was prepared by covalently binding polyacrylic acid (PAA) on Fe3O4 superparamagnetic nanoparticles (13.2 nm) via carbodiimide activation. The maximum weight ratio of PAA to Fe3O4 was 0.12 (i.e., average of two PAA molecules on a magnetic nanoparticle). The magnetic nano-adsorbent possessed a high ionic exchange capacity of 1.64 meq g(-1) and was efficient for the recovery of lysozyme. The lysozyme could be completely adsorbed in 0.1 M phosphate buffer at pH 3-5 and completely desorbed in NaSCN solution (>1 M) within 1 min, and retained 95% activity after adsorption/desorption. In addition, the adsorption behavior followed the Langmuir adsorption isotherm with a maximum adsorption amount of 0.224 mg mg(-1) and a Langmuir adsorption equilibriumconstant of 10 ml mg(-1) at 25 degreesC. The change of enthalpy at 15- 35degreesC was -4.2 kJ ml mol(-1) mg(-1).