Increasing concentrations of anions of the Hofmeister series decrease the activity of highly purified glutamate dehydrogenase (EC 1.4.1.2.) from Pi- sum s a t i v u m L. The extent of the inactivation, as estimated by the ion concentration which causes a 50% t r a n s f o r m a t i o n of the native form to the low activity form of the enzyme ( a p p r o x i m a t e l y " h a l fm a x i m a l activity "), follows the ranking C1 < F < B r -< N O 3 < S C N -. Sulfate has a slightly activating effect. A t salt concentrations higher than 1 M (with S C N -higher than 200mM), the activity decreases to a value from 3-6 % of the initial activity and remains then stable over a wide range of higher anion concentrations. F r o m kinetic investigations it is seen that the treatment of the enzyme with anions decreases the affinity for the cosubstrate N A D + and the substrate L -g l u t a m a t e (KM-Values increased) and also increases the dissociation constant for N A D Β§ . The salt induced inactivation is reversible by dilution. F r o m a mathematical treatment of the kinetic d a t a of the inactivation, it is seen that increasing concentrations of the anions exert cooperative effects on the inactivation process.