A　new　electrochemical　immunosensor　was　designed　for　the　determination　of　carcinoembryonic　antigen　(CEA)　with　sensitivity　enhanced　by　using　nanogold　poly(o-phenylenediamine)-manganese　dioxide　organic-inorganic　hybrid　nanostructures　(GNPM)　as　nanotags　and　peroxidase　mimics.　Initially,　mesoporous　poly(o-phenylenediamine)-manganese　dioxide　(PPD-MnO2)　hollow　microspheres　were　synthesized　by　an　inorganic/organic　interfacial　polymerization　technique.　Then　gold　nanoparticles　were　assembled　onto　the　surface　of　PPD-MnO2,　which　were　used　for　the　labelling　of　the　anti-CEA　detection　antibody　(pAb(2)).　The　prepared　GNPM　nanotags　were　characterized　using　transmission　electron　microscopy　(TEM),　field　emission　scanning　electron　microscopy　(FESEM),　UV-vis　absorption　spectroscopy,　N-2　adsorption-desorption　isotherm　measurements　and　Fourier　transform　infrared　spectroscopy　(FTIR).　The　assay　was　carried　out　with　a　sandwich-type　immunoassay　format　in　pH　5.5　acetic　acid-buffered　saline　solution　containing　2.5　mmol　L-1　H2O2.　Experimental　results　indicated　that　the　electrochemical　immunosensor　exhibited　a　wide　dynamic　range　from　0.01　to　80　ng　mL(-1)　towards　the　target　CEA　with　a　detection　limit　(LOD)　of　6.0　pg　mL(-1).　The　immunosensor　also　displayed　a　good　stability　and　acceptable　reproducibility　and　selectivity.　In　addition,　the　methodology　was　evaluated　by　assaying　10　clinical　serum　samples,　providing　a　good　relationship　between　the　electrochemical　immunosensor　and　the　commercialized　electrochemiluminescent　(ECL)　method　for　determination　of　CEA.