A method for determining the activity of an angiotensin-converting enzyme is disclosed comprising the steps of mixing a liquid containing (i) a synthetic substrate X-hippuryl-L-dipeptide having the formula ##STR1## wherein X represents OH, NH.sub.2 or N(CH.sub.3).sub.2, and when R.sub.1 is H, R.sub.2 is H, and when R.sub.1 is ##STR2## R.sub.2 is CH.sub.2 CH(CH.sub.3).sub.2, (ii) hippuricase and (iii) 4-aminoantipyrine, with a liquid containing an angiotensin-converting enzyme; measuring colorimetrically the concentration of the quinonimine dye which is formed by adding an oxiding agent to the above mixture; and calculating the activity of the angiotensin-converting enzyme from the concentration of the quinonimine dye.