and mercuric chloride (0 1 mM) but not by sulphydryl reactants. 3. The reaction is characterized by a marked increase in iodine-staining power and P-amylolysis limit but oc-(I-÷ 4)-glucosidic linkages in glycogen are not attacked. Enzyme action is incomplete, only the outer oc-(1-> 6)-glucosidic linkages being hydrolysed. 4. Isoamylase differs from the plant and animal debranching enzymes [R-enzyme and amylo-(1-+ 6)-glucosidase] since both amylopectin and glycogen are substrates. 5. Purified isoamylase has no action on maltose, isomaltose or maltotriose. 6. Isoamylase is also present in baker's yeast.