An immunoassay has been developed for the determination of alpha-fetoprotein (AFP). It is based (a) on the use of a detection reaction mediated by manganese dioxide nanoparticles (MnO2 NPs) because their good stability at room temperature, and (b) on tyramine signal amplification (TSA). The MnO2 NPs acts as an artificial peroxidase that causes the conversion of TMB to give a colored product, and the tyramine-triggered reaction is used for signal amplification to improve the detection limit. Combined with immuno-magnetic separation and enrichment, the response of this AFP immunoassay is linear from 6.25-400 ng mL(-1), with a detection limit of 22 pg mL(-1) (S/N = 3). This immunoassay was successfully applied to the quantification of AFP in serum samples, and gave excellent accuracy compared with the clinical results from a local hospital. The LOD and stability of this assay are better than those of the standard horseradish peroxidase-based ELISA. The strategy presented here is conceived to have a wider scope in that it may be extended to various other immunoassays.