Beef liver glutamate dehydrogenase is one of a number of allosteric, or regulatory, enzymes which are known to demonstrate reversible self-aggregation in vitro. In this report we present evidence that aggregation plays an important role in the allosteric control of this enzyme. Quasi-elastic light scattering spectroscopy is used in conjunction with bio-chemical determinations of enzyme activity in order to quantitatively characterize the relation between aggregation and enzyme activity. A mathematical model is presented which successfully predicts this experimentally observed relation and elucidates the specific role of aggregation in the allosteric regulation of this enzyme. We find that the net effect of the aggregation is: (1) to cause the allosteric transition of the enzyme from inactive to active form to occur at a lower level of allosteric activation, where the level of allosteric activation is a measure of the relative concentrations of allosteric activators and inhibitors; and (2) to make this allosteric transition a more abrupt function of the level of allosteric activation. This finding has important implications for the functioning of this enzyme as a control element in protein metabolism.