An　immunosensor　for　detection　of　microcystin.　(leucine.　arginine)　(MCLR)　was　constructed.　Chemical　co-precipitation　method　was　used　to　synthesize　ferriferrous　oxide　magnetic　nanoparticles　(Fe3O4　MNPs).　The　Fe3O4　MNPs　were　modified　with　3-aminopropyltriethoxysilane　(APTS)　and　succinic　anhydride　(SAH)　in　turn　to　generate　carboxyl-functionalized　core-shell　MNPs　(Fe3O4　@　APTS　center　dot　SAH　MNPs),　which　were　characterized　by　transmission　electron　microscope　(TEM),　hysteresis　loop,　X-ray　photoelectron　spectroscopy　(XPS)　and　Fourier　transform　infrared　spectroscopy　(FTIR),　respectively.　The　Fe3O4　@　APTS　center　dot　SAH　MNPs　were　modified　onto　the　surface　of　a　home-made　magnetic　glassy　carbon　electrode　(MGCE).　Anti-MCLR　antibody　(anti.　MCLR)　was　immobilized　on　the　modified　MGCE　by　coupling　reaction　with　ethyl-3-(dimethyl　aminopropyl)　carbodiimide　(EDC)　and　N-hydroxysuccinimide　(NHS).　Subsequently,　bovine　serum　albumin　(BSA)　was　used　to　block　the　non.　specific　adsorption　sites　of　the　electrode.　Direct　competitive　immunoassay　format　was　adopted.　In　the　presence　of　horseradish　peroxidase-conjugated　MCLR　(MCLR.　HRP),　MCLR　in　solution　was　determined　by　differential　pulse　voltammetry　(DPV).　Under　the　optimized　conditions,　MCLR　could　be　detected　within　a　wide　linear　range　of　0.05-100　mu　g/L　with　a　detection　limit　of　0.01　mu　g/L　(S/N　=　3).　Moreover,　the　immunosnssoe　exhibited　good　reproducibility,　stability　and　selectivity.　The　immunosensor　was　applied　for　the　determination　of　MCLR　in　real　water　samples　with　the　recoveries　from　94.3%　to　99.5%.