Human skin chymotrypsin-like proteinase, human neutrophil cathepsin G, rat mast cell chymase, and rat salivary gland tonin are cell-derived serine proteinases of similar size with specificity for amino acids of aromatic residues. Each enzyme was examined for its ability to convert angiotensin I to angiotensin II and to cleave a panel of synthetic substrates. Skin chymotryptic proteinase, cathepsin G, and tonin cleaved the phe₈- his₉ bond of angiotensin I and converted angiotensin I to angiotensin II without further degradation. In contrast, chymase formed relatively small amounts of angiotensin II because it preferentially cleaved the tyr₄-ile₅ bond of angiotensin I. The rank order of angiotensin I converting activity was skin ehymotryptic proteinase > tonin > cathepsin G > chymase. The K_m and K_cat for angiotensin I conversion by the human skin enzyme were 6.6 × 10^-5 M and 50 s^-1, respectively. The angiotensin I converting activity of human skin chymotryptic proteinase is equal to or greater than the peptidyl dicarboxypeptidase angiotensin-converting enzyme. Substrate specificities of each enzyme were further distinguished by use of benzoyl-L-tyrosine ethyl ester. A limited immunologic characterization of each enzyme was performed with monospecific goat antiserum to cathepsin G and chymase by Ochterlony gel diffusion. Each antiserum gave a precipitin line against its respective immunogen without evidence of cross-reactivity against the other enzymes. Human skin chymotryptic proteinase, cathepsin G, and tonin provide unique pathways for the generation of angiotensin II in tissue and may be of significance in regulation of biologic processes of the tissue microenvironment. The kinetic constants of the human skin chymotryptic proteinase for angiotensin I conversion, are consistent with the potential to carry out a reaction of physiologic importance.