Studies have been conducted on three types of glycogen storage diseases: (1) the molecular heterogeneity of muscle phosphorylase deficiency (type V) has been shown by demonstration of the presence of cross-reacting immunological material in one out of two cases; (2) the presence of cross-reacting material has been demonstrated in 18 tissue samples from patients with amylo-1,6-glucosidase deficiency (type III); (3) the existence of pathological variants of amylo-1,4-glucosidase (type II) has been shown by electrophoresis and thermal denaturation. In addition, the molecular distinction between ‘neutral’ and ‘acid’ amyloglucosidase has been provided using electrophoretic and immunological techniques.