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The plasma globulins which produce the vasoactive polypeptides, bradykinin, kallidin and angiotensin, have been compared. After incubation of plasma with kallikrein and exhaustion of its kallidinogen, subsequent incubation with trypsin did not result in formation of bradykinin, showing that bradykininogen had also been exhausted and suggesting that kallikrein and trypsin use the same substrate. Kallidin and bradykinin formation was not prevented by acid-treatment of plasma, though heat-denatured substrate produced kallidin less readily. Kallikrein could exhaust plasma bradykininogen without affecting levels of angiotensinogen. Following nephrectomy of dogs, plasma angiotensinogen levels rose whereas bradykininogen levels did not. These results confirm the belief that bradykininogen differs from angiotensinogen but not from kallidinogen.