The compound 3-(5-bromoindolyl)-[beta]-D-glucopyranoside (I) was successfully used as the potential chromogen to demonstrate giucosidase in fresh tissue from rats, mice and guinea pigs. Tissue, frozen at -70[degree]C, cut in cryostat at -20[degree]C and 5 [mu], was incubated in a substrate solution containing 4 mg of (I) in 1 ml dimethylformamide, 5 mg PVP in 1 ml water and 14 ml acetate buffer, 0.2 M, pH 5.4. The glucosidase in tissue hydrolyzed (I) to form 5-bromoindoxyl (II) which readily formed the highly insoluble chromogen 5, 5[image]-dibromoindigo (III). The final reaction product (III) could be readily demonstrated at sites of enzyme activity in cells and tissues. The highest activity was present in the preputial gland and the small intestine as well as certain reticulum cells. Intense activity was present in acini and ducts of the preputial glands. Enzyme activity increased in older acini and ducts. Intense activity was seen in reticulo-endothelial cells of villous stalks of small intestine. Peripheral reticulum cells in spleen and lymph node showed reaction. Most other organs showed only traces and some were negative. Enzyme activity is destroyed by heat, ferri-ferrocyanide, formalin and Cu sulphate, and diminished by high concentrations of (I). Saccharo-l-4-lactone which is an inhibitor of glucuronidase did not effect the enzyme. The pH optimum was 5.4.