A serine protease active on insoluble elastin at neutral pH has been isolated from human aortic media employing a Lima-bean trypsin inhibitor - Sepharose column. It is also hydrolyzed Suc (Ala)3 pna and casein but was found inactive against Benzoyl-Tyr-pna and Benzoyl-Arg-pna. Its apparent molecular weight as determined by SDS-PAGE was 22,300 Daltons. It differs from other elastases of human origin (human pancreatic elastase-1, human pancreatic elastase-2, human leucocyte elastase) on the basis of amino-acid composition and immunological specificity.