The kidney localization and glomerular deposition of soluble immune complexes in mice were greater and more persistent following the intravenous administration of complexes prepared with reduced and alkylated antibodies than following the administration of complexes prepared with intact antibodies. The increased glomerular deposition following the administration of complexes prepared with reduced and alkylated antibodies was associated with the persistence of circulating complexes composed of more than two antigen and two antibody molecules (Haakenstad AO, Mannik M:Lab Invest 35:283, 1976). The deposition of immune complexes in glomeruli, as detected by immunofluorescence, appeared to precede the detection of mouse C3 in glomerular deposits following the administration of both preparations of complexes. The deposition of mouse C3 was more intense and persisted longer in mice receiving complexes containing reduced and alkylated antibodies than in mice receiving complexes containing intact antibodies. The ultrastructural studies indicated that both preparations of complexes initially localized as electron-dense material in endothelial cell fenestrae and in the subendothelial space of the glomerular capillary loops and subsequently accumulated in the mesangial matrix between mesangial cells. The material persisted in the mesangium of mice receiving complexes with reduced and alkylated antibodies, whereas it was removed from the mesangium of mice receiving complexes with intact antibodies. The mechanism for removal of complexes from the mesangial matrix was not defined, but it did not appear to occur through phagocytosis by the mesangial cell.