Angiotensin I converting enzyme (kininase II, peptidyl dipeptidase, ACE) was purified by reverse immunoadsorption from a membrane fraction of the human kidney. ACE is very likely a transmembrane peptidase. Treatment of the membrane-bound enzyme with trypsin releases a low mol. wt. fragment (greater than 10,000), which is probably the anchor peptide inserted into the plasma membrane. Antibody to ACE was used to localize it in the CNS where it is bound to plasma membrane of neuroepithelial cells in structures such as the globus pallidus or substantia nigra. Radioimmunoassay indicated that ACEs of endothelial, epithelial and neuroepithelial origin are immunologically identical. Direct radioimmunoassay also showed that there is a strong negative correlation between plasma enzyme level and pulmonary diffusing capacity of sarcoid patients. Finally, in addition to various peptides, homogeneous human ACE cleaves fluorogenic substrates where the C-terminal amino acid is replaced with nitrobenzylamine.