Immune complex deposits found on the subepithelial side of the glomerular basement membrane (GBM) are characteristic of membranous glomerulonephritis. In order to clarify the mechanism of subepithelial immune complex deposition, we investigated the manner of localization of preformed immune complexes (IC) composed of native ferritin (NF) and anti NF antibody (aNFab) in a 40-fold antigen great excess using the system of passive serum sickness nephritis of mice. The following results were obtained. (1) The ferritin particles in the GBM of animals given NF alone, as controls, were mainly restricted to the lamina rara interna (LRI) with very few penetrating the entire depth of the GBM. (2) In animals given the IC, greater numbers of ferritin particles were seen, in comparison with the control animals, not only in the lamina densa (LD) but also in the lamina rara externa (LRE). (3) Immunoelectron microscopy (IEM) demonstrated that each NF particle was accompanied by aNFab, which was found to be peroxidase positive material, throughout the GBM. (4) On the grounds that ICs were prepared in Ag great excess, it is suggested that ICs seen in the GBM are composed of a low avidity antibody with a low Ag/Ab ratio, probably Ag1Ab1. From these results, it is suggested that circulating ICs are necessary to give rise to IC deposition on the GBM and that NF and aNFab exist together as ICs throughout the GBM. We concluded that subepithelial IC deposits arise from the circulation, and that after GBM trapping, circulating small-sized IC can pass through the LD to reach the subepithelial side of GBM.