Ferric chloride, when boiled with ammonium thiocyanate, ammonia and cacodylic acid, is converted into a fine anionic iron colloid which consists of 1-1.5 nm electron dense granules and gives a distinct Prussian blue reaction. This colloid allows light and electron microscopic detection of ionized cationic sites in tissues at a pH range of 4.0-9.0. Light and electron micrographs of the rat kidney stained with this colloid are demonstrated. These micrographs indicate that the foot processes of glomerular podocytes and the brush border of the proximal convoluted tubule contain positively-charged groups in their intracellular matrices, and that the foot process plasma membrane fronting the Bowman's capsular space has positively-charged groups. The latter finding, together with our previous study (Murakami et al., 1986), suggests the co-existence of cationic and anionic sites on this foot process surface.