The role of charge-charge interactions between cationic immune complexes and the anionic sites on the glomerular basement membrane was examined. For this purpose, soluble immune complexes at fivefold antigen excess were prepared with human serum albumin and cationized rabbit antibodies to this protein. When unrelated cationic proteins, protamine sulfate or cationized rabbit serum albumin, were given 1 min before the cationized immune complexes, glomerular immune deposits did not form. Cationic immune complexes allowed to deposit in glomeruli could readily be displaced by protamine sulfate or cationized rabbit serum albumin injected 1 min after the immune complexes. If the same cationic molecules were injected 1 hr after the immune complexes, the complexes could not be displaced from glomeruli. In contrast, cationic complexes that were deposited in glomeruli in the presence of a very high degree of antigen excess in circulation to prevent their condensation into larger complexes in glomeruli were readily displaced at 1 min and 1 hr with protamine sulfate or with cationized rabbit serum albumin. On the basis of these results, we concluded that the initial binding of cationic immune complexes to glomeruli occurs by charge-charge interactions. Once the immune complexes in glomeruli condense to larger deposits, forces other than charge-charge interactions are responsible for their retention in glomeruli.