NEW MATERIAL:Peptides having amino acid sequence structures of formulas. I, II, III, etc., and having angiotensin transferase-inhibiting activities.

USE: An agent for diagnosing, preventing and treating hypertension, and a vasodepressor.

PREPARATION: The angiotensin transferase-inhibiting activity-having peptides of formulas I, II and III are prepared e.g. as follows; treating swine blood plasma with a semi-permeable membrane such as an ultra-filtration membrane, adding the passed components to a strong acid cation exchange resin column, washing the column with de-ionized water, eluting the adsorbed substances with a 2N ammonium hydroxide solution, concentrating the eluted solution under vacuum to remove ammonia, gel-filtering the concentrated solution, collecting fractions having angiotensin transferase-inhibiting activity, further fractionating the collected fractions by an ion exchange gel filtration method, collecting a highly physiologically active fraction and subsequently applying a reversed phase high performance liquid chromatography treatment to the fraction for purification.