Human lung membrane-bound neutral metallo-endopeptidase (NME; EC 3.4.24.11) has been purified; this enzyme occurred in two forms, NME-I and NME-II. The total NME activity was purified 2,143-fold with the final specific activities for NME-I and NME-II being 750 and 1,124, respectively. The two NME forms were resolved in the final purification step involving ion exchange; in all earlier steps including gel filtration and affinity chromatography (phenyl sepharose) both forms behaved similarly and eluted simultaneously. NME-I and NME-II both had a Mr value of 97,000, and neither form dissociated into subunits. Catalytic actions of NME-I and NME-II upon bradykinin were identical; the Gly4-Phe5 and Pro7-Phe8 bonds of bradykinin were cleaved with the final hydrolytic products for each enzyme being the tetrapeptide, Arg-Pro-Pro-Gly, the tripeptide, Phe-Ser-Pro, and the dipeptide, Phe-Arg. The intermediate products were the heptapeptide, Arg-Pro-Pro-Gly-Phe-Ser-Pro, and the pentapeptide, Phe-Ser-Pro-Phe-Arg. Neither NME-I nor NME-II were inhibited by the angiotensin-converting enzyme inhibitor, captopril. Both enzymes were inhibited by phosphoramidon, dithiothreitol and EDTA. Other peptidase inhibitors and heavy metals were not effective NME inhibitors. Both NME-I and NME-II cleaved angiotensin-I at the Pro7-Phe8 bond, and substance-P at the Glu6-Phe7 bond, with the latter being much slower than the former.