Alpha-Glucan phosphorylase (EC 2.4.1.1.) was purified from sweet potato roots. Apparently homogeneous preparations obtained are partially degraded products from phosphorylase, as judged from the results of molecular weight determination, NH-2-termini analysis and pyridoxal-5'-P assay. Phosphorylase is shown to be degraded in the crude extract from sweet potato. The degradation is partly suppressed by EDTA and by salts and is accelerated by reducing agents. It is proposed that sweet potato phosphorylase in its intact form has a similar molecular structure and similar properties to the white potato enzyme. Both plant phosphorylases are preferentially cleaved by protease near the middle of their polypeptide chains without much loss of enzyme activity.