Two siblings with the clinical features of glycogen storage disease are described. Glucagon tolerance tests revealed a nonsignificant rise in blood sugar above fasting levels, whereas intravenous administration of galactose resulted in a normal increase in blood glucose. Amyloglucosidase (debrancher enzyme) activity was absent in liver and muscle, as assayed by incorporation of U-14C glucose and by release of glucose from a phosphorylase limit dextrin. Incorporation of U-14C glucose, although reduced, occurred in leukocytes and was normal in erythrocytes. Transferase activity was present in muscle and liver but absent in leukocytes. Assay of enzyme activity in blood cells only, as a means of diagnosing type III glycogenosis, may be misleading.