Inactivation of animal amylases is complicated by the presence of neutral salts which are necessary for the enzyme to exhibit its maximal activity. Liquefaction of starch and the subsequent production of reducing sugar are both due to the activity of the same enzyme, and differences previously reported can be explained as due to variations in the nature and amounts of accompanying substances present. Salivary amylase is not inactive when entirely free from neutral salts, but the presence of salts alters the pH optimum. Salt-free amylase at its optimum (pH 6.0) shows about 40% of the activity of chloride-amylase at its optimum (pH 6.6-6.8). NaCl forms a compound with amylase, but phosphate, acetate, and sulphate have no effect other than that due to alteration in pH. Nitrate-amylase shows the same pH optimum as chloride-amylase, but its activity is no greater than that of the salt-free enzyme. Chlorate-amylase is similar to nitrate amylase. Bromide- and iodide-amylase also show maximal activity at pH 6.6-6.8, but are less active than chloride-amylase. The difference between the activities of amylase and its salt compounds is not due to difference in affinity for the substrate but to the alteration of dissociation constants of the enzyme when united to salts. In support of this hypothesis, picric acid and phosphotungstic acid inactivate amylase by combining with the basic group of the enzyme. Amylase is inactivated by nitrites, though not to the same extent as invertase. The inactivation in this case is increased by presence of salts which form compounds with the amylase, but not by salts with which no combination occurs. Inactivation by Hg, on the other hand, is more marked with the salt-free enzyme. Pancreatic amylase shows the same pH optima and activity in the presence or absence of salts as salivary amylase under similar conditions, and the 2 enzymes are identical; slight differences in inactivation phenomena are ascribed to the presence of different impurities.