With existing methods, estimates of endogenous renin in human serum or plasma differ widely, in part due to technical differences. In a new relatively simple and less time-consuming method the essential technical details are the use of purified, angiotensinase-free substrate added to angiotensinase-free serum, incubation for a prolonged period, and control assays of increasing quantities of a standard preparation of human renin added to the serum. Under these conditions, the yield of angiotensin increases linearly with the concentration of the added renin, and the concentration of endogenous renin in the serum can be calculated from the slope of the line, which represents the quantity of angiotensin produced per unit of added renin, and the intercept, which represents the amount of angiotensin produced without added renin. A precise assay of renin expressed in terms of an internationally acceptable standard preparation of human renin can thus be achieved.