A sensitive direct human renin radio-immunoassay has been developed for clinical use. The antigen source was human renal renin purified from Haas' preparation by pepstatin-C6-sepharose affinity chromatography, this was used to prepare a specific human renin antibody. The radio-immunoassay was performed by the double antibody technique using the delayed tracer addition method. Standard curves were obtained over the range 0.2-8.0 ng/ml. Dilution curves of human renal renin and human plasma were superimposable on the standard curve. Both active and inactive renin were detected by this method, and measurements correlated well with total renin activity after trypsin activation. Intra- and inter-assay coefficients of variance were 4.6% and 5.1%, respectively. Renin concentration was higher in patients with renovascular hypertension (1.97 +/- 0.38 ng/ml, mean +/- s.d., n = 10, P less than 0.01), but lower in primary aldosteronism (0.66 +/- 0.16 ng/ml, n = 13, P less than 0.01) compared with essential hypertension (1.38 +/- 0.34 ng/ml, n = 12). This method provides a new tool for the investigation of the renin-angiotensin system in man.