An assay for angiotensin I and its use for the determination of plasma-renin activity is described. The assay employs 125I labelled antibody to angiotensin I. Rabbits were immunized with angiotensin I adsorbed on carbon particles, the antibody purified with immunoadsorbent and labelled with 125iodine. In determining plasma-renin activity endogenous renin-substrate was used; the renin releasing angiotensin I during incubation. Ethylene-diaminetetra-acetic acid (EDTA) and 1:10 phenanthroline were included in the incubation mixture to prevent destruction of angiotensin I or its conversion to angiotensin II. Assay of the generated angiotensin I was carried out directly on plasma. The measurement of the reaction rate of renin added to plasma allowed the determination of plasma-renin concentration as well as the plasma-renin activity.