Abstract: Disclosed is an iron colloid-labeled antibody wherein an iron colloid is bound to IgG or an antigen binding fragment thereof without a loss of antibody specificity. An iron colloid stabilized with cacodylic acid or the salt thereof (ferric cacodylate colloid) is preferable.This iron colloid-labeled antibody is prepared by mixing the iron colloid stabilized with cacodylic acid or the salt thereof with IgG or the antigen binding fragment thereof at a pH of about 7.4 to 7.5.The iron colloid-labeled antibody is used for the histochemical detection of specific constituents of cells or tissued under both a light microscope and an electron microscope, which gives distinct stained figures in each case.