A　new　sandwich-type　electrochemical　immunoassay　was　developed　for　the　detection　of　human　IgG　using　doubly-encoded　and　magnetic　redox-active　nanoparticles　as　recognition　elements　on　the　surface　of　a　glassy　carbon　electrode　modified　with　anti-IgG　on　nanogold　particles.　The　recognition　elements　were　synthesized　by　coating　magnetic　Fe3O4　nanoparticles　with　Prussian　blue　nanoparticles　and　then　covered　with　peroxidase-labeled　anti-IgG　antibodies　(POx-anti-IgG)　on　Prussian　blue　nanoparticles.　The　immunoelectrode　displays　very　good　electrochemical　properties　towards　detection　of　IgG　via　using　double-encoded　magnetic　redox-active　nanoparticles　as　trace　and　hydrogen　peroxide　as　enzyme　substrate.　Its　limit　of　detection　(10　pmol·L-1)　is　10-fold　better　than　that　of　using　plain　POx-anti-IgG　secondary　antibodies.　The　method　was　applied　to　the　detection　of　IgG　in　serum　samples,　and　an　excellent　correspondence　with　the　reference　values　was　found.　©　2010　Springer-Verlag.