The rate of formation of angiotensin was investigated for the reaction of angiotensinase-free human renin, at various states of purity, with angiotensinogen, in pooled, angiotensinase-free human serum. The velocity constant of this reaction and the Michaelis constant of the renin-substrate complex were determined. The concentration of angiotensinogen and of endogenous renin in human serum was also determined and, on the basis of these results, a procedure was designed for the indirect assay of human renin. This was carried out under a wide range of experimental conditions, for periods of incubation ranging from 10 min to 18 hours, for various concentrations of substrate, and for renin concentrations varying from 0.000025 to 0.20 unit/ml. The absence of angiotensinase made possible the prolonged incubation (18 hours) of a minute quantity of renin with a large amount of the renin substrate. The resultant formation of large amounts of angiotensin permitted its accurate assay in the dog.