Abstract Angiotensin I-converting enzyme was purified 1500-fold from human plasma utilizing an immunoadsorbent affinity gel prepared by coupling antibody to baboon lung angiotensin I-converting enzyme to CNBr-activated Sepharose 4B. The enzyme was eluted from the gel using 2 m magnesium chloride, pH 5.8. Subsequent hydroxylapatite and Sephadex G-200 chromatography yielded 2.6 mg of homogenous enzyme with a specific activity of 40 units/mg with hippuryl- l-histidyl- l-leucine as substrate from 48 liters of plasma. Use of the immunoadsorbent allowed the 48 liters of plasma to be processed in one-half the time it previously took to process 2 liters of plasma by other methods. This protocol enables us to obtain sufficient amounts of enzyme for structural studies that were previously impossible because of insufficient amounts of enzyme.