The effect of the charge of circulating immune complexes on glomerular localization was studied in a model of passive serum sickness. Preformed immune complexes of heterogeneous or restricted charge, prepared in vitro from isoelectrically focused or chemically modified proteins, were injected intravenously into mice. The distribution of immune complexes in the kidney was compared by immunofluorescence and electron microscopy. Cationic but not anionic or electrophoretically heterogeneous immune complexes gave rise to diffuse localization in the glomerular basement membrane. The binding in subepithelial and subendothelial sites correlated with the known distribution of structural anionic sites. The observations suggest that electrostatic interactions between fixed anionic sites and immune complexes may be an important factor in glomerular trapping. Alternative mechanisms based on initial localization of excess free cationic antigen cannot be completely excluded and are also considered.