The purification of angiotensin I-converting enzyme which is known to act as a key enzyme in the control of blood pressure through the Renin-Angiotensin-Aldosterone system, was achieved using affinity chromatography as the principal purification step. The angiotensin I-converting enzyme was partially purified from hog lung homogenates by acidification, DEAE-cellulose chromatography and ultrafiltration. The kinetic properties and the effect of sodium chloride have showed the known characteristics of angiotensin I-converting enzyme. Therefore, it was confirmed that the enzyme was angiotensin I-converting anzyme. An effective affinity gel was prepared on Sepharose 6B matrix using succinylL-proline competitive inhibitor as a ligand and spacer arm was introduced by 1,6-hexanediamine between the matrix and a ligand. With this affinity column, the enzyme preparation showed high degree of purity and good purification yield.