Using tri-sodium citrate as reducer, stable silver nanoparticles the size of about 20 nm were prepared by the microwave high pressure procedure with simplicity and rapidity. At pH 9.0, sliver nanoparticle was used to label goat anti-human IgG (GIgG) to obtain an immuno-nanosilver resonance scattering spectral probe (AgGIgG) for IgG. In pH 6.0 buffer solution and in the presence of polythylene glycol (PEG) and KCl, the immune reaction of IgG with AgGIgG took place, the silver nanoparticles released from AgGIgG produced aggregations, and the resonance scattering intensity at 485 nm (I(485 nm)) was enhanced greatly. The influence factors such as pH value, buffer solution volume, concentration of AgGIgG, KCl, PEG-4000, PEG-6000, PEG-10000 and PEG-20000, incubation temperature and time were considered, respectively. Under the conditions of 0.40 mL of pH 6.0 phosphate buffer solution, 1.20 mL of 9.8 microg x mL(-1) AgGIgG, 0.20 mL of 20% PEG-6000, 0.50 mL of 10% KCl, and ultrasonic irradiation for 25 min at room temperature, the increased intensity deltaI(485 nm), was proportional to the IgG concentration (c(IgG)) from 0.004 to 0.48 microg x mL(-1), with a detection limit of 2.4 ng x mL(-1). The regress equation was deltaI485 nm = 76.8c(IgG) + 4.7. The effect of foreign substances such as 20 microg x mL(-1) Ni2+, Fe2+, Pb2+ and BSA,60 microg x mL(-1) Cu2+, Ca2+ and HSA,60 microg x mL(-1) Mg2+ and Mn2+, 320 microg x mL(-1) Zn2+, glucose and urea on the deltaI(485 nm) was examined, respectively. Results showed that there was no interference. This assay showed high sensitivity and good selectivity for quantitative determination of IgG in human serum with satisfactory results. The analytical results were in agreement with that of the reference results.