1. An elastolytic enzyme has been isolated from dog granulocyte leukocytes. The purification procedure included preparation of the granula fraction, chromatography on Sephadex G-75 and ion-exchange chromatography on SP-Sephadex C-50 at pH 6.0. 2. The elastase isolated was homogeneous in analytical disc electrophoresis and showed in sodium dodecylsulfate electrophoresis a single protein component with the molecular weight of 24800. The enzyme lacked tyrosine and lysine and the N-terminal amino acid was phenylalanine. No carbohydrate or sialic acid were detected. 3. The dog granulocyte elastase showed similar activities as human granulocyte elastase on elastin and fibrin. The Km value for 3-carboxypropionyl-L-alanyl-L-alanyl-L-alanyl-p-nitroanilide was 2.50 mM and the pH optimum 8.5. The elastase preparation obtained was 99.5% active as judged from active-site titration. 4. The enzyme is a cationic protein and shows pronounced trailing on agarose gel electrophoresis. 5. A monospecific antiserum against the purified enzyme was produced in rabbits.