Cathepsin G, a human lysosomal neutral protease, converts angiotensin I to angiotensin II and cleaves angiotensin II from a plasma protein substrate. Experiments were designed that identified and characterized cathepsin G substrate as human angiotensinogen. A total of 2, 5, and 10 micrograms of purified substrate, incubated with 2 microL of partially purified human renin (2 Goldblatt units/mg) for 60 min at 37 degrees C, generated 2, 9, and 22 pmol of angiotensin I. Cathepsin G substrate and renin substrate activities copurified during Affi-Gel Blue affinity chromatography, hydroxylapatite chromatography, phenyl-Sepharose chromatography, and S-200 gel filtration. Disc gel electrophoresis of 10 micrograms of purified protein gave a single band containing both activities. The amino-terminal sequence contained the covalent structure of angiotensin I and was Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu-Val-Ile-His-X-Glu-Ser-Thr-Cys-Gl u-. Reduced and unreduced angiotensinogens were subjected to sodium dodecyl sulfate gel electrophoresis, and each gel showed two bands of Mr 65 000 and 62 000. The isoelectric point of the Mr 65 000 form was pH 4.5-4.3 and the Mr 62 000 form was pH 4.9. Functional, structural, and physiochemical evidence demonstrates that the substrate of cathepsin G is angiotensinogen. Thus, human neutrophils may utilize angiotensin I or angiotensinogen as substrate for angiotensin II generation. The granulocyte-angiotensin system does not require renin or converting enzyme and may function as a mobile effector pathway which modulates tissue blood flow and/or vascular permeability.