Purified red kidney bean (Phaseolus vulgaris) amylase inhibitor forms a 1:1 stoichiometric complex with porcine pancreatic α‐amylase leading to complete loss of enzyme activity on starch. Rate of complex formation is pH dependent and is maximal at pH 5. The rate constants for complex formation, as measured by loss of amylase activity, were 2.85 × 104M‐1sec‐1at pH 6.9 (ionic strength of 0.918) and 2.55 × 105M‐1sec‐1at pH 5 at 30°C. At pH 6.9, rate of complex formation was 4.8 times faster at 0.918 ionic strength as compared with the rate at 0.138 ionic strength. At 30°C, pH 6.9 and ionic strength of 0.168 the dissociation constant of the enzyme‐inhibitor complex was determined to be 3.5 × 10‐11M. The rate constant for dissociation of the complex was calculated to be 8.7 × 10‐8sec‐1under the same conditions. The rate constant for complex formation, at ionic strength of 0.168, was 1.1 × 104M‐1sec‐1at 370and 9.77 × 102M‐1sec‐1at 25.7°C. The calculated activation energy for complex formation is 39.5 kcal/mole suggesting a rate‐controlling conformational change. Oxidation of the carbohydrate moiety of the glycoprotein inhibitor caused complete loss of activity. Maltose, a competitive inhibitor of α‐amylase, bound as readily to the enzyme‐inhibitor complex as to free α‐amylase. Trypsinized α‐amylase, although still able to bind to Sephadex, did not bind inhibitor. The experiments with maltose and trypsinized amylase suggest the inhibi