The adsorption of bromelain from an aqueous solution by polyacrylic acid (PAA)-bound iron oxide magnetic nanoparticles was studied. The magnetic composite nanoparticles were shown to be efficient for the separation of bromelain. Except at pH < 3, the adsorption of bromelain increased with the decrease in solution pH and reached almost 100% at pH 3-5. The adsorbed bromelain could be desorbed by the addition of KCl and complete desorption was achieved at pH 7 when [KCl] > 0.6 M. The adsorption behaviour followed the Langmuir isotherm with a maximum adsorption amount of 0.476 mg/mg and a Langmuir adsorption equilibrium constant of 58.4 ml/mg at pH 4 and 0.1 M phosphate. In addition, it was notable that both the adsorption and desorption of bromelain were quite fast and could be completed in about 1 min due to the absence of internal diffusion resistance. Bromelain retained 87.4% activity after adsorption/desorption. (C) 2003 Elsevier Ltd. All rights reserved.