ADP-glucose pyrophosphorylase has been isolated in homogeneous form from an Escherichia coli B mutant, AC70R1, derepressed in the synthesis of glycogen synthetic enzymes. The enzyme has been found to be identical with the wild type enzyme with respect to kinetic properties, molecular weight, and immunological reactivity. The AC70R1 enzyme is composed of four identical subunits of molecular weight of approximately 50,000. This is based on the findings that: (a) gel electrophoresis under denaturing conditions shows only one component; (b) tryptic mapping shows only enough peptides to account for a single polypeptide subunit; (c) digestion with carboxypeptidase B releases stoichiometric amounts of arginine; and (d) NH2-terminal sequencing shows a single sequence for the first 27 residues.