Hog kidney homogenate was fractionated initially following the steps presented by De La Haba et al. (1959) for the purification of cathepsin C from beef spleen. The preparation was further fractionated by gel filtration using Sephadex G 200, starch gel and immunoelectrophoresis. The following enzymes were identified in the fractions obtained: Cathepsin C, which liberated ammonia from glycyl-phenylalanine amide and naphthylamine from glycyl-phenylalanyl-β-naphthylamide, pH optimum at 5.0, was activated by cysteine and inhibited by sulfhydryl reagents. An aminopeptidase, which liberated first of all glycine from glycyl-phenylalanine amide and glycyl-phenylalanyl-β-naphtylainide and after that ammonia and naphthylamine, respectively, hydrolysed numerous amino acid naphthylamides, pH optimum at 7.0–7.5, was activated by Co++ and inhibited by EDTA. A peptidase, which liberated glycine from glycyl-phenylalanine amide and naphthylamide, did not hydrolyse amino acid naphthylamides, maximally active at neutral pH, was inhibited by EDTA. Several esterases, two in gel filtration, 5–6 in starch gel and immunoelectrophoresis, hydrolysing 5-bromoindoxyl acetate. The activities were sensitive to E 600. Both the studies on the characteristics of these activities as well as starch gel and immunoelectrophoretic studies support the view that none of the esterase activities is identical with cathepsin C. Cathepsin C, on the other hand, does not hydrolyse significantly 5-bromoindoxyl acetate and, consequently, this substrate can not be used to demonstrate cathepsin C histochemically. Glycyl-phenylalanyl-β-naphthylamide is recommended as a new sensitive chromogenic substrate for cathepsin C in biochemical studies in which the role of the aminopeptidase (s) can be adequately excluded but cannot be used in the histochemical demonstration of this enzyme.