This article reviews some properties of human leucocyte elastase. This 30 kDa glycoprotein formed of 218 amino acid residues, is a serine proteinase which cleaves proteins at Val-X, Ala-X, Leu-X or Met-X bonds. Leucocyte elastase solubilizes fibrous elastin and also degrades other extracellular matrix proteins. It hydrolyses and inactivates a number of plasma proteins. Synthetic substrates are more convenient than elastin to measure elastase activity. A large number of natural and synthetic inhibitors of leucocyte elastase have been described. The former include alpha 1-proteinase inhibitor or alpha 1-antitrypsin, inter-alpha-inhibitor, alpha 2-macroglobulin, bronchial and cervical mucous inhibitor and a number of animal and plant proteins. Numerous synthetic inhibitors with therapeutic potentials have been designed. The efficiency of an inhibitor depends, among others, upon its rate of association with the enzyme and upon the stability of the enzyme-inhibitor complex. Elastase probably plays a physiological function in neutrophil migration, phagocytosis and tissue remodeling. It apparently plays a pathological role in pulmonary emphysema, rheumatoid arthritis, infections and inflammation. The pathogenic role of leucocyte elastase is best understood in emphysema.