Poly-L-lysine with molecular masses of 3.3-290 kDa increased the amidolytic activities of leukocyte elastase and cathepsin G at low concentration, but had little effect on the activities of pancreatic elastase, alpha-chymotrypsin, plasmin and thrombin. Highly purified cathepsin G was obtained from column of EAH Sepharose 4B or Suc-L-Tyr-D-Leu-D-Val-pNA-Sepharose (affinity chromatography) by elution with poly-L-lysine solution (0.4 mg/ml, molecular weight (MW.) 290000 or 2.2 mg/ml, MW. 3300). Leukocyte elastase, adsorbed to Suc-L-Tyr-D-Leu-D-Val-pNA-Sepharose, was not eluted with poly-L-lysine solution. The amino acid composition of purified cathepsin G has been determined.