Abstract 1. 1. Ehrlich-Lettré ascites carcinoma cells of the mouse were incubated with [ i- 14C]- and [6- 14C]glucose. The intracellular Glc-6- P was isolated and degraded. After incubation in the presence of iodoacetate and cyanide, a significant amount of C-6 of the added glucose is found in position i, and about the same amount of C- i is found in positions 4, 5 and 6 of the isolatd Glc-6- P. 2. 2. The transfer of C-6 of the added glucose to position i in the Glc-6- P was followed by further oxidation of the carbon to CO 2 via the pentose-phosphate cycle. As the rate of lactic acid formation and glucose utilization decreased, the rate of the tranfer increased. 3. 3. Phosphofructokinase of the ascites tumor cells catalyzed the formation of Fru-6- P and ATP from Fru- i ,6-P 2 and ADP. The fructose- i,6-diphosphate activity of the cells is, if present at all, very low. 4. 4. It is suggested that, in the presence of iodoacetate, the glycolytically formed triose phosphates are, after equilibirum by the triose-phosphate isomerase, converted to Glc-6- P by reversal of the glycolytic reaction. As the formation of Fru-6- P from Fru- i ,6-P 2 is catalyzed by phosphofructokinase, and not by fructose i,6-diphosphatase, an exchange between C- i and C-6 of Glc-6- P is possible without net loss of ATP.