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The phosphofructokinase of the red cells of rats is exquisitely sensitive to inhibition by ATP. The apparent Ki for ATP is decreased with lower pH values and fructose-6-phosphate concentrations. 2,3-Bisphophoglycerate, which is present in the red cells of man and many mammals, inhibits the phosphofructokinase in a synergistic manner with ATP. The inhibitory effect of phosphoenolpyruvate is biologically insignificant. It may be estimated that the enzyme would be practically completely inactive under cellular conditions without the action of positive effectors. There exist a multitude of positive effectors. Significant activations under cellular conditions are exercised by AMP, glucose 1,6-bisphosphate and inorganic phosphate. Fructose 1,6-bisphosphate activates to a limited extent and ADP only at alkaline pH values. All positive effectors produce a decreased inhibition by ATP and increased affinity of the enzyme for fructose 6-phosphate. AMP is the activator with the highest affinity, while glucose 1,6-bisphosphate may be considered as a baseline activator owing to its sluggish turnover. The data may be fitted in a qualitative manner to the model of Monod, Wyman and Changeux. It is possible that the negative anionic effectors act on the same site. [ABSTRACT FROM AUTHOR]