We have developed and validated a new enzyme-kinetic method for measurement of renin concentration (PRC) in human plasma, based on radioimmunoassay of angiotensin I generated during incubation of plasma and excess sheep or ox renin substrate. Angiotensin I breakdown during incubation is prevented by the presence of anti-angiotensin I serum. The assay does not require prior extraction of renin, is technically simple, and is sufficiently sensitive to measure subnormal renin levels. With minor modifications both “active” and “total” renin may be measured. Assay results have been calibrated with the International Standard Renin. PRC measured by this technique correlates significantly with angiotensin I and II, plasma renin activity, and with the PRC method previously used by us.