The enzymatic study on an angiotensin I converting enzyme (ACE) inhibitor of (2R, 4R)-2-(2-hydroxyphenyl)-3-(3-mercaptopropionyl)-4-thiazolidinecarboxylic acid (SA 446) was described. ACE was purified from rabbit lung, and the specific activity of final preparation was 22.5 U/mg protein with Hip-His-Leu as substrate. The concentration of SA 446 producing 50% inhibition of ACE activity was 6 nM. The inhibition mode was competitive with K₁ of 2.2 nM. The inhibitory potency did not change even in the presence of a relatively large amount of Zn²⁺ or Co²⁺. SA 446 in the concentration of 0.1 mM did not produce 50% inhibition on the other enzymes including zinc-enzyme such as carboxypeptidase A, B, N, leucine aminopeptidase and alkaline phosphatase. These results indicated that SA 446 was an extremely potent and specific inhibitor of ACE. The structural characteristics of SA 446 were also demonstrated by the study on analogous compounds.