Abstract: Objective To prepare a kind of mag neticiron-dextran nanoparticles that was coated with anti.coli O157:H7 IgG,and to establish the optimum conditions of magnet ic nanoparticles applicatio n for the detection of E.coli O157:H7.Methods Magneticiron-dextran nano particles were prepared by reacting a mixture of ferric and ferrousions with dextran polymers under alkaline conditions.The particles were coated with antiserum against E.coli O157:H7 by the periodat eoxidation-borohydr ide reduction procedure.F inally,the immunomagnetic nanoparticles were prepared for the detection.Results E.coli O157:H7 could be isolated from samples within 15 min with the sensitivity of 101CF U/ml oreven less.In the presence of 10⁸CFU/ml other organisms,the sensitivity was 10¹-10² CF U/ml.Nonspecfic bacter ium sho wed very weak influence on detection results.Conclusion Isolation of target bacteria by immunomagnetic nanoparticles is an efficient method with high sensitivity and specificity.