A　colorimetric　immunoassay　was　built　for　determination　of　carcinoembryonic　antigen　(CEA)　based　on　papain-based　colorimetric　catalytic　sensing　system　through　the　use　of　glucose　oxidase　(GOx).　In　the　presence　of　GOx,　glucose　was　catalytically　oxidized　to　produce　H2O2.　Through　the　assistance　of　papain　(as　a　peroxide　mimetic　enzyme),　the　signal　came　from　the　oxidative　color　development　of　3,3＇,5,5＇-tetramethylbenzidine　(TMB,　from　colorless　to　blue)　catalyzed　by　the　generated　H2O2.　Herein,　a　sandwich-type　immunoassay　was　built　based　on　GOx　as　labels.　As　the　concentration　of　CEA　increased,　more　GOx-labeled　antibodies　specifically　associate　with　target,　which　leaded　to　more　H2O2　generation.　Immediately　following　this,　more　TMB　were　oxidized　with　the　addition　of　papain.　Accordingly,　the　absorbance　increased　further.　As　a　result,　the　concentration　of　CEA　is　positively　correlated　with　the　change　in　absorbance　of　the　solution.　Under　optimal　conditions,　the　CEA　concentration　was　linear　in　the　range　of　0.05–20.0　ng/mL,　and　the　limit　of　detection　(LOD)　reached　37　pg/mL.　The　papain-based　colorimetric　immunoassay　also　exhibited　satisfactory　repeatability,　stability,　and　selectivity.　©　2024　Elsevier　B.V.