This review summarizes data on structure of muscle glycogen phosphorylase b and the role of the cofactor pyridoxal 5"-phosphate in catalysis and stabilizing the native conformation of the enzyme. Specific attention is paid to the stabilizing role of pyridoxal 5"-phosphate upon denaturation of phosphorylase b. Stability of holoenzyme, apoenzyme, and enzyme reduced by sodium borohydride is compared.