Endothelin (ET) is a 21-residue potent vasoconstrictive peptide produced by vascular endothelial cells and formed from its precursor, big endothelin (big ET), by endothelin-converting enzyme (ECE). This paper describes the cloning and functional expression of a cDNA encoding a human ECE from human umbilical vein endothelial cells (HUVEC). Human ECE consists of 758 amino acid residues and has high homology to rat and bovine ECE. Immunoblot analysis using a monoclonal antibody risen against rat lung ECE showed the presence of immunoreactive protein in membrane fraction prepared from both HUVEC and COS-1 cells transfected with human ECE cDNA. Both COS-1 cells expressing human ECE and its membrane fraction converted big ET-1 most efficiently among big ETs.