We developed a sensitive and specific heterogeneous enzyme immunoassay for measuring angiotensin I (AI) or renin (EC 3.4.99.19) activities. Linking AI under carefully controlled conditions by means of N-succinimidyl-3-(2-pyridyldithio)propionate to highly purified beta-D-galactosidase (EC 3.2.1.23) produced a well-defined conjugate in high yield. The procedure is simple and consists of three steps: (a) incubate 0.5 mL of plasma with 100 microL of inhibitor solution for 1 h to generate renin activity; (b) incubate this with labeled AI for 1 h at 37 degrees C; and (c) do a double-antibody precipitation. Bound beta-D-galactosidase activity is determined, with O-nitrophenyl-beta-D-galactoside as substrate, by measuring the liberated O-nitrophenol at 410-420 nm. The analytical range of the assay extends from 62.5 to 4000 ng/L, and renin activities so measured in plasma correlate well with those measured by an established radioimmunoassay (n = 35, r = 0.97, intercept -0.6, and slope 1.44).