Isoamylases identified by use of agar or cellulose acetate electrophoresis and quantified by densitometory of the zymogram. In normal human serum there are 2 types of isoamylases present, namely pancreatic (P₁) and salivary (S₁) isoamylase. In fifty sera of normal subjects, average of amylase activity of isoamylase P₁ and isoamylase S₁ were 54 and 57 somogyi units, respectively. In the sera of patients with acute pancreatitis, abnormal of isoamylases were found to appear. When compared with normal serum, three additional isoamylases were found in sera of these patients. They were named P₂, P₃ and P₄, P₄ was found in serum of a patient with most severe type of acute pancreatitis. When the inflammatory process was going to subside, P₄, P₃ and P₂ disappeared one by one according to this order. Similary abnormal isoamylases such as S₂ and S₃ were detected in the serum of patients with acute parotitis. These abnormal isoamylases were proved not to be the subunits of amylase by use of gel filtration. The zymogram of liver homogenate was identical with that of serum, indicating that the liver is not the organ to produce amylase.