We recently reported that urinary excretion rates of angiotensinogen (U_AGT) provide a specific index of intrarenal renin-angiotensin (Ang) system (RAS) status in AngII-dependent hypertensive rats. When this is shown to be applicable to human subjects, a diagnostic test to identify those hypertensive patients most likely to respond to a RAS blockade could provide useful information to allow a mechanistic rationale for selecting an optimized approach to the treatment of hypertensive subjects. However, simple and accurate methods to measure human angiotensinogen (hAGT) are unavailable at this time. To perform future human subject studies, we developed antibodies and a sensitive and specific quantification system for hAGT using a sandwich ELISA. We raised 2 antibodies against hAGT: one is mouse monoclonal and the other is rabbit polyclonal. The standard curve of this ELISA exhibited a high linearity from 0.31 to 20 ng/mL. The correlation coefficient was over 0.99. Plasma angiotensinogen concentrations of healthy volunteers were from 28 to 71 µg/mL (N=10). The ratio of UAGT to urinary creatinine concentration was from 5.0 to 30 µg/g (N=7). The %CVs for intra-assay and the %CV for inter-assay were from 4.4 to 5.5% and from 4.3 to 7.0%, respectively. This ELSIA system had no crossreactivity with major proteins in proteinuric urine samples such as human albumin, immunoglobulin, or transferrin. Moreover, the crossreactivity of the system with angiotensin peptides was also negligible. The development of this hAGT ELISA will be a useful tool to investigate the relationship between U_AGT and reactivity to antihypertensive drugs in hypertensive subjects.