A new method for isolation of leukocyte serine proteinases has been developed. Elastase (EC 3.4.21.37) and cathepsin G (EC 3.4.21.20) have been isolated from dog neutrophils and purified to homogeneous state. The results of inhibitor analysis indicate that the enzymes belong to the group of serine proteinases. Some physical and chemical characteristics of the purified enzymes have been determined. The molecular weights of the enzymes are 24.5-26 kD for the elastase and 23.5-25.5 kD for the cathepsin G. The cathepsin G is a glycoprotein, while the elastase molecule lacks carbohydrate components. The cathepsin G exhibits a broad pH optimum of catalytic activity in the range of 7.0-9.0; the pH optimum for the elastase is 8.0-8.5. The Michaelis constant of the elastase for N-t-Boc-L-alanine p-nitrophenyl ester is 0.10 mM; the Michaelis constant of the cathepsin G for N-benzoyl-L-tyrosine ethyl ester is 0.42 mM.