Mouse kidneys were perfused with Krebs-Ringer bicarbonate buffer (KRB) containing native, anionic horse spleen ferritin or various cationized derivatives, and the glomerular localization of the probe molecules determined by electron microscopy. Ferritins cationic with respect to the medium (KRB, pH 7.45) accumulated in the subendothelial layers of the glomerular basement membrane (GBM) in amounts far exceeding those observed with anionic ferritins, the degree being greater for the more cationized derivatives. Strongly cationized ferritins, in addition permeated the full thickness of the GBM in considerable amounts, but appeared to be retarded from entry into the urinary spaces at the level of the filtration slits. Very strongly cationized derivatives adhered to glomerular endothelium and GBM and formed aggregates in the outer layers of the latter. The results suggest that intrinsic negative charges are present in the GBM and endothelium, and that the barrier function of the glomerular capillary wall may be ascribed in part to its electrophysical properties.