Angiotensin I was coupled very easily with a-β-galactosidase (EC 3. 2. 1. 23) using a novel coupling reagent, N-(meta-maleimidobenzoyloxy) succinimide. No decrease in enzyme activity was observed during coupling procedure. Angiotensin I-β-D-galactosidase conjugate was proved to possess enough enzyme activity and immune reactivity for enzyme immunoassay of angiotensin I. By the competitive binding test, 1.5pg to 50pg of angiotensin I could be detected with a fairly good precision. The cross reactivity of angiotensin II, angiotensin III, 1-Sar-8-Ileu-angiotensin II and deamino-dicarba-Arg-vasopressin were compared to angiotensin I by the present enzymeimmunoassay and also usual radioimmunoassay using the same antiserum to angiotensin I. The results indicated that these peptides were distinguished clearly from angiotensin I but the specificities to the tested analogues by enzyme immunoassay were different from those by radioimmunoassay. This difference was diminished by an addition of a small amount of angiotensin I in the system for enzyme immnoassay. The present method was applied to measure plasma renin activity of dogs and humans. The results were compared to those obtained by standard radioimmunoassay. The both methods showed a fairly good coincidence (r=0.94, dog, r=0.90, human).