A novel enzyme-free sandwich electrochemical immunoassay with an ultrahigh sensitivity was developed for detection of alpha-fetoprotein (AFP, as a model analyte) using carbon nanotube-enriched gold nanoparticles (CNT-AuNPs) as nanolabels/nanocatalysts on anti-AFP/glutaraldehyde/thionine-modified glassy carbon electrodes (GCEs). The assays were carried out in a pH 8.0 acetic acid-buffered solution containing 6 mM p-nitrophenol (NP) and 6mM NaBH(4) after the formation of the sandwich-type immunocomplex. Initially, the NP molecules were reduced to p-aminophenol (AP) by the catalysis of the immobilized gold-nanoparticle labels on the CNT-AuNPs with the aid of NaBH(4), then the generated AP molecules were electrochemically oxidized to p-quinone imine (QI) by an electron mediator of thionine, and then the oxidized QI molecules were reduced back to APs by NaBH(4). The redox cycling of AP and QI continuously increased the signaling, leading to a high sensitivity. Compared with individual gold-nanoparticle labels, the immunosensor using CNT-AuNPs as labels displayed a wider linear range of 8.0×10(-7)-2.0×10(2) ng/mL with a lower detection limit (LOD) of 0.8 fg/mL AFP at a signal-to-noise ratio of 3, which was lower 6 orders than that of commercially available ELISA. Intra-and inter-assay coefficients of variation were below 10%. In addition, the assay was evaluated with clinical serum samples, and no significant differences at the 5% confidence level were encountered in the analysis of real samples between the proposed immunoassay and commercially available Roche 2010 Electrochemiluminescent Automatic Analyzer for determination of AFP.