Depletion of energy stores of human red cells decreases the maximum transport capacity, Jm, for glucose transport to a value one-third or less of that found in red cells from freshly drawn blood. There is no change in Km. Hemolysis and resealing of red cells with ATP or ADP reverses the decrease in Jm. The maximum effect occurs at concentrations of ATP in the normal range for red cells, however, there is little effect from ADP concentrations in its normal range in freshly drawn red cells. Hemolysis and resealing with ATP gives an increase in Jm and an increase in differential labeling by photolytic labeling with tritiated cytochalasin B. Most of the activation is lost after a second hemolysis-reseal without ATP but about 25% of the activation remains.