Trypsin, when incubated with human plasma protein either undenatured or denatured by pretreatment with acid or alkali or by heat, produced an angiotensin II-like pressor substance instead of the depressor bradykinin which has been reported to be formed from various plasma fractions and trypsin. This reaction may be due to the activation of prorenin by trypsin, but this is rather unlikely since it appears that angiotensin II rather than I is formed. It seems to be more likely a direct production of the angiotensin hormone from the substrate, and we propose to call tentatively the active product "tryptensin".