Gold nanoparticle were modified with goat-IgG and used as a mass-probe for dot blotting. On the spots of a blotting membrane, where anti-IgG antibody was cast, the IgG-modified gold nanoparticles were deposited through immunochemical interaction. Laser desorption/ionization mass spectrometry desorbed the gold nanoparticles as Au+ ions, and indicated the presence of 20 pg of anti-IgG antibody. The detection limit of the mass spectrometry was 1,000 times better than that of nakedeye observation of the color of gold nanoparticles.