Fe3O4/poly(styrene-co-maleic anhydride) core-shell composite microspheres, suitable for binding enzymes, were prepared using magnetite particles as seeds by copolymerization of styrene and maleic anhydride. The magnetite particles were encapsulated by polyethylene glycol, which improved the affinity between the magnetite particles and the monomers, thus showing that the size of the microspheres, the amount of the surface anhydrides, and the magnetite content in the composite are highly dependent on magnetite particles, comonomer ratio, and dispersion medium used in the polymerization. The composite microspheres, having 0.08-0.8 mum diameter and containing 100800 mug magnetite/g microspheres and 0-18 mmol surface-anhydride groups/g microsphere, were obtained. Free a-amylase was immobilized on the microspheres containing reactive surface-anhydride groups by covalent binding. The effects of immobilization on the properties of the immobilized alpha-amylase [magnetic immobilized enzyme (MIE)] were studied. The activity of MIE and protein binding capacity reached 113,800 U and 544.3 mg/g dry microspheres, respectively. The activity recovery was 47.20%. The MIE had higher optimum temperature and pH compared with those of free a-amylase and showed excellent thermal, storage, pH, and operational stability. Furthermore, it can be easily separated in a magnetic field and reused repeatedly. (C) 2004 Wiley Periodicals, Inc.