Layered core-shell bimetallic silver-gold nanoparticles were prepared by coating Au layers over Ag seedsby a seed-growth method. The composition of Ag100-xAux particles can vary from x) 0 to 30. TEM andSEM images clearly show that the bimetallic nanoparticles are of core-shell structure with some pinholes onthe surface. Strong surface-enhanced Raman (SER) signals of thiophenol and p-aminothiophenol have beenobtained with these colloids. It was found that the SERS activity of aggregated colloids critically depends onthe molar ratio of Ag to Au. With the increase of the Au molar fraction, the SERS activity enhances first andthen weakens, with the maximal intensity being 10 times stronger than that of Ag colloids. The AgcoreAushellnanoparticles were then labeled with monoclonal antibodies and SERS probes and used for immunoassayanalysis. In the proposed system, antibodies immobilized on a solid substrate can interact with the correspondingantigens to form a composite substrate, which can capture reporter-labeled AgcoreAushell nanoparticles modifiedwith the same antibodies. The immunoreaction between the antibodies and antigens was demonstrated by thedetection of characteristic Raman bands of the probe molecules. AgcoreAushell bimetallic nanoparticles, as anew SERS active and biocompatible substrate, will be expected to improve the detection sensitivity ofimmunoassay.