In view of the long-recognized role of the liver in the control of the blood sugar concentration in animals (Soskin, Essex, Herrick & Mann, 1938; Soskin & Levine, 1952; Cahill, Ashmore, Renold & Hastings, 1959) and the appreciation of the part played by glucose 6-phosphatase (D-glucose 6-phosphate phosphohydrolase, EC 3.1. 3.9) in the production of glucose by the liver (for reviews, see Ashmore, 1959; Weber, 1959), the study of the nature of the enzymic system for the phosphorylation of glucose by the liver has started rather late. Various experimental procedures (see, for example, Slein, Cori & Cori, 1950; Long, 1952; Renold, Hastings, Nesbett & Ashmore, 1955; Bekina & Petrova, 1957; Vester, 1959) gave glucose-phosphorylating activities too low to account for the observed rates of conversion of glucose into carbon dioxide and glycogen (Renold et al. 1955) by liver slices.