Positively charged antibodies mediate enhanced deposition of circulating immune complexes at the glomerular basement membrane. The presented experiments demonstrate that when soluble immune complexes were prepared with a mixture of antibodies containing 10 to 25% cationic antibodies, then noncationic antibodies in the complexes were deposited in mouse glomeruli. One or two cationic antibodies in each immune complex sufficed for deposition of the complexes. Proof for this was obtained by two kinds of experiments. First, the injected immune complexes were prepared in Ag excess from mixtures of radiolabeled noncationic rabbit antibodies to human serum albumin (HSA) and unlabeled cationized rabbit antibodies to HSA, thus permitting the specific quantitation of the deposition of noncationic antibodies in glomeruli because of the presence of cationized antibodies within the same complexes. As a control experiment, immune complexes prepared only with noncationic antibodies resulted in very little deposition in kidneys over the same time period. Second, detection of the localization of the noncationic antibody in deposits in glomeruli by immunofluorescence microscopy was accomplished using immune complexes prepared with mixtures of noncationic goat antibodies to HSA and cationized rabbit antibodies to HSA. Thus, the synthesis of a small population of cationic antibodies during the immune response may lead to the formation of circulating immune complexes with enhanced propensity for deposition in glomeruli in patients with SLE or other immune complex diseases.