Angiotensin-converting enzyme (ACE) was purified about 7000 times from human lung tissue obtained at thoracotomy. After solubilization with Triton X-100 and sonication, ion exchange DEAE cellulose chromatography and Sepharose 4B gel filtration were performed. After gel filtration a 5-6 fold increase in purity was achieved by neuraminidase treatment of the protein and recycling over DEAE cellulose. Purity was established in SDS electrophoresis and on electrofocusing 125I-labelled purified protein and these procedures indicated a molecular weight of about 150,000 and pI value of 4.5, respectively. The purified protein split Angiotensin I and this action was inhibited by Captopril (Squibb 14,225), specific inhibitor of ACE (kininase II). The Km value for the synthetic substrate hippuryl-histidyl-leucine was 3.7 X 10(-4) mol/l. The IC50 of Captopril when inhibiting human lung ACE action on the same substrate, was 4.5 X 10(-9) mol/l.