Cathepsin G (CTSG) is a neutral proteinase secreted by neutrophil granulocytes. It belongs to the family of serine proteases. Cathepsin G is a glycoprotein, which contains 1% carbohydrate. The activity assay of recombinant rat CTSG was measured by its ability to cleave the peptide substrate Suc-Ala-Ala Pro-Phe-pNA. The reaction was performed in 160 mM Tris-HCl, 1.6 M NaCl, pH 7.4 (Assay Buffer). The CTSG was diluted to different concentrations and the reaction was initiated by adding 50 ul CTSG to 50 ul 1 mM substrate and then read at 405 nm in kinetic mode for 5 minutes. The specific activity of recombinant rat CTSG is >70 pmol/min/μg.