* This work has been supported in part by a research grant from the Nutrition Foundation. t Fellow of the Netherlands Organization for Pure Research (Z.W.O.). 1 T. Baranowski, B. Illingworth, D. H. Brown, and C. F. Cori, Biochim. et Biophys. Acta, 25, 16, 1957. 2 C. F. Cori and B. Illingworth, these PROCEEDINGS, 43, 547, 1957. 3 M. Cohn and G. T. Cori, J. Biol. Chem., 175, 89, 1948. 4The following abbreviations are used: Gi-P,a-glucose-1-phosphate; Pinorg, inorganic orthophosphate; Pyrs-P, pyridoxal-5-phosphate; Tris, tris-(hydroxymethyl)-amino methane; EDTA, ethylenediaminetetraacetic acid; AMP-5',5'-adenylic acid; ATP, adenosine triphosphate. 5 The authors are grateful to Dr. Luis Glaser for preparing this compound enzymatically from uniformly labeled C14-starch. 6 C. F. Cori, G. T. Cori, and A. A. Green, J. Biol. Chem., 151, 39, 1943. 7 (NH4)2SO4 saturated at 5°. The pH of the solution was adjusted with NH3 and determined with the Beckman pH meter set at 100, after dilution of the cold solution to 0.25 saturation. 8 Na thioglycolate was selected because apophosphorylase a could be readily dissolved at room temperature in solutions of this sulfhydryl compound and repeatedly crystallized in the cold. Usually, crystallization of the apoenzyme did not occur when cysteine-glycerophosphate was substituted for thioglycolate. 9 N. B. Madsen and C. F. Cori, J. Biol. Chem., 223, 1055, 1956.