M-Aminophenol gives a more rapid and more complete blockage of the Schiff reaction of aldehydes produced in tissue by oxidation with periodic or chromic acid than does aniline. Moreover, the phenol thus attached to aldehyde sites in tissue can then be azo coupled with alkaline solutions of fresh or stabilized diazonium salts. When so used at pH 8, fast black K, C.I. 37190, shows glycogen, mucins, basement membranes, muscle stroma, fungus cell walls, etc. stained black, on a somewhat grayish pink to red background.