The　hepatotoxic　microcystins,　especially　microcystin-LR　(MC-LR),　are　causing　serious　problems　to　public　health　and　fisheries.　We　describe　here　a　label-free　amperometric　immunosensor　for　rapid　determination　of　MC-LR　in　water　sample.　The　sensor　was　prepared　by　immobilizing　antibody　on　a　gold　electrode　coated　with　L-cysteine-modified　gold　nanoparticles.　The　stepwise　self-assembly　of　the　immunosensor　was　monitored　and　characterized　by　means　of　electrochemical　impedance　spectroscopy　and　differential　pulse　voltammetry.　A　0.60　mmol　L-1　solution　of　hydroquinone　was　used　as　the　electron　mediator.　The　immunosensor　was　incubated　with　MC-LR　at　25　°C　for　20　min,　upon　which　the　differential　pulse　voltammetric　current　changed　linearly　over　the　concentration　range　from　0.05　to　15.00　μg　L-1,　with　a　detection　limit　of　20　ng　L-1.　The　developed　biosensor　was　used　to　determine　MC-LR　in　spiked　crude　algae　samples.　The　recovery　was　in　the　range　from　95.　6　to　105%.　This　method　is　simple,　economical　and　efficient,　this　making　it　potentially　suitable　for　field　analysis　of　MC-LR　in　crude　algae　and　water　samples.　©　2011　Springer-Verlag.