The enhancement in sensitivity for an electrochemical immunoassay by the autocatalytic deposition of Au 3+ onto gold nanoparticles has been studied. By coupling the autocatalytic deposition with square-wave stripping voltammetry, enlarged gold nanoparticles labeled on goat anti-rabbit immunoglobulin G (GaRIgG-Au) and, thus, the rabbit immunoglobulin G (RIgG) analyte could be determined quantitatively. A variety of variables, such as concentration of AuCl 4 −, the reducing agent used, the duration of autocatalytic deposition, and parameters for the stripping analysis were optimized. From a calibration graph over a broad dynamic range of concentrations (1–500 pg mL −1; R 2 = 0.9975) a very low detection limit, 0.25 pg mL −1 (1.6 fM), which is three orders of magnitude lower than that obtained by a conventional immunoassay using the same gold nanoparticle labels was obtained; this finding confirms applicability and effectiveness of our method of enhancing the sensitivity of gold nanoparticle label-based sandwich immunoassays. The reliability of this method was confirmed by the rather low values of RSD (2.82%, n = 11; 2.44%, n = 9) obtained for assays of a blank solution and for 0.02 ng mL −1 RIgG solution, respectively.