Angiotensin-converting enzyme (ACE) from human liver was first purified 9000-fold by chromatofocusing with 22% yield. The enzyme had a specific activity of 10 U/mg. The enzyme molecular weight was 150000, as determined by electrophoresis in a 7.5% polyacrylamide gel. The enzyme pI determined by chromatofocusing was 4.2-4.3. KM of human liver ACE, measured using hippuryl-L-histidyl-L-leucine and N-benzyloxycarbonyl-L-phenylalanyl-L-histidyl-L-leucine as substrates, was 5 mM and 0.1 mM, respectively. Human liver ACE was inhibited by SQ 20881 with IC50 equal to 1.8 X 10(-8) M.