A sensitive solid phase enzyme-linked immunosorbent assay (ELISA) has been developed using two anti-human renin monoclonal antibodies, which were shown to bind both human as well as primate renin at two different epitopic sites. One monoclonal antibody (3-36-16) was used to coat each well of a 96 well microtitre plate in which renin, contained in 5 to 20 ul of plasma, was allowed to react. Plates were then incubated with the gamma globulin (gamma G) fraction of a rabbit anti-human renin serum followed by development with sheetp anti-rabbit gamma G conjugated to alkaline phosphatase. Quantification was carried out by the addition of the alkaline phosphatase substrate, p-nitrophenylphosphate, which produced a colorimetric reaction. The sensitivity of the assay is 25 pg ml-1. The method recognises both active and inactive renin from plasma, kidney, amniotic fluid and chorionic cells. Plasma renin can be measured within 6 hours when values are greater then 150 pg ml-1 or within 24 hours when plasma values are less than 150 pg ml-1. The ELISA has already been used to measure total immunoreactive renin in plasma obtained from patients with several forms of hypertension. The values ranged from 25 pg ml-1 in patients with primary aldosteronism to as large as 60 ng ml-1 in a patient with a renin-secreting tumour.(ABSTRACT TRUNCATED AT 250 WORDS)