Chromatographically purified from leucocytes of purulent human sputum and lyophilized. The lyophilized, salt-free, soluble powder is stable for over a year when stored at 5 C. The protein is greater than 95percent pure by SDS-PAGE and is free of cathepsin G, MPO and lysozyme. 800-900 units per mg of protein on the substrate Suc-Ala-Ala-Ala-pNA. 15000-19000 units per mg protein on the substrate MeO-Suc-Ala-Ala-Pro-Val-pNA. One unit will hydrolyze 1 nanomole of substrate per minute at pH 7.5 and 25 C. The following diluents and concentrations are recommended: 1) 1 mg/ml in 0.05 M Na Acetate pH 5.0 containing 0.1 M NaCl. Stable 7 days at 5 C (in ice). 2) 10 mg/ml in 50percent glycerol 50percent 0.02 M Na Acetate pH 5. Stable at least 60 days at -20 C with 20 cycles between -20 C and 5 C. 3) May be dissolved at 10 mg/ml in 2 mM HoAc and re-lyophilized in smaller quantities. Stability is better in acidic (pH 5.0) than basic buffers. Inactivation may occur at pH 8.0 due to autolysis and/or proteolysis by trypsin such as bovine pancreatic trypsin. Proteolysis can be inhibited by benzamidine. Autolysis and proteolysis can be inhibited by elastatinal. However, elastatinal is also an inhibitor of elastase. Low pH protects the elastase from both autolysis and proteolysis.