The nature of chloride ion dependency on the activity of angiotensin-converting enzyme was studied by kinetic analysis using hippuryl-Gly-Gly and N-Cbz-Gly-Pro as the substrate analogs. The enzyme showed a strong chloride ion dependency on its full activity. When the activity of the enzyme was demonstrated by varing the concentration of the peptide substrate at different fixed concentration of chloride ion, the resulting reciprocal plots showed a typical converging pattern indicating a possible involvement of a ternary complex between the enzyme, the peptide substrate and chloride ion. Chloride ion can be treated as a nonessential activator which may increase the rate of enzymatic reaction by decreasing the apparent Km value. The calculated values for hippuryl-Gly-Gly and N-Cbz-Gly-Gyl-Pro were 0.07 and 0.46, respectively. Based on the kinetic data obtained it is suggested that chloride ion may have a role as an allosteric effector in the binding of the enzyme and substrate.