In sodium acetate–acetic acid buffer solution, Au, Ag, Pt, Pd, Fe 3 O 4 , and Cu 2 O nanoparticles have catalytic enhancement effect on the reduction of Cu 2+ by ascorbic acid to form large copper particles that exhibit a strong resonance scattering peak at 610nm. Those nanocatalytic reactions were studied by the resonance scattering spectral technique, and smaller nanogold exhibited stronger catalytic enhancement effect in pH 4.2 sodium acetate–acetic acid buffer solution. The resonance scattering intensity at 610nm increased linearly with the concentrations of 0.02 to 1.60, 0.040 to 1.20, and 0.12 to 4.70nM nanogold in sizes of 5, 10, and 15nm with detection limits of 0.010, 0.030, and 0.10nM, respectively. An immunonanogold-catalytic resonance scattering bioassay was established, combining the immunonanogold-catalytic effect on CuSO 4 –ascorbic acid reaction with the resonance scattering detection technique. As a model, 0.03 to 7.5ngml −1 immunoglobulin G can be assayed by this immunonanogold-catalytic resonance scattering bioassay with a detection limit of 0.015ngml −1 .