The hemolysate partially purified from human red cells was demonstrated to be capable of synthesizing fructose-2,6-bisphosphate (F-2,6-P2) from fructose-6-phosphate in the presence of adenosine triphosphate (ATP) indicating that human red cells contain fructose-6-phosphate,2-kinase. The effect of F-2,6-P2 on the rate-limiting enzymes of glycolysis, ie, hexokinase, phosphofructokinase (PFK), and pyruvate kinase, has also been examined. PFK was activated by this metabolite and the half- maximum activation was obtained at a concentration of 10(-7) mol/L. Neither hexokinase nor pyruvate kinase was affected by F-2,6-P2. These results suggest that human erythrocytes may contain this metabolite as one of the positive effectors for PFK.