The effect of I- and C1- on the enzymatic degradation of elastin pancreatic elastase has been studied. At low halide concentrations both ions show a marked stimulation of elastolysis, I- being significantly more effective than Cl-. Kinetic plots of enzyme activity indicate that anions enhance the susceptibility of the substrate to enzymatic and non-enzymatic degradation. If the halide concentration is raised inhibition of elastolysis occurs starting at 30 mM NaCl and 300 mM NaI. This inhibitory effect is explained by a direct attack of the halides leading to increasing destabilization and inactivation of the enzyme.