A　simple　colorimetric　immunoassay　for　quantitative　monitoring　of　brevetoxin　B　on　a　functionalized　magnetic　bead　by　using　glucose　oxidase　(GOx)/antibrevetoxin　antibody-labeled　gold　nanoparticle　as　the　signal　transduction　tag　was　developed.　The　assay　was　carried　out　on　the　basis　of　GOx-controlled　sulfite-to-sulfate　chemical　conversion　with　a　silver(I)-3,3＇,5,5＇-tetramethylbenzidine　[Ag(I)-TMB]　system.　Initially,　the　sulfite　was　used　as　an　inhibitor　of　Ag(I)　to　hinder　the　color　development　of　TMB　due　to　the　formation　of　insoluble　silver　sulfite.　Accompanying　H2O2　generation　with　GOx-catalyzed　glucose,　the　sulfite　was　converted　into　the　sulfate,　thus　resulting　in　the　colorless-to-blue　change.　Under　the　optimal　conditions,　the　absorbance　decreased　with　increasing　brevetoxin　B　from　0.5　to　200　ng/kg　with　a　detection　limit　of　0.1　ng/kg　(ppt).　The　precision　and　specificity　were　acceptable.　Furthermore,　the　methodology　gave　results　matching　well　with　the　referenced　brevetoxin　ELISA　kit　for　monitoring　of　spiked　Musculista　senhousia　samples.