Isoelectric precipitation at pH 5.0 and the use of the inhibitors, turanose, maltose and citrate, enabled the diagnosis of Pompe's disease to be made in dextran-isolated leucocytes using 4-methylumbelliferyl-alpha-D-glucopyranoside as substrate. These techniques were unnecessary with lymphocytes as the deficiency of acid alpha-glucosidase could be shown directly.