In the course of screening for new aminopeptidase M inhibitors which were expected to be analgesic, immunopotentiating, or anti-metastatic agents, the novel synthetic substance MR-387C[(2S,3R)-3-amino-2-hydroxy-4-phenylbutanoyl-L-valyl-L-prolyl-L-leucine] (M.W. 504 daltons) was obtained. It was competitive with the substrate and had an $IC_{50}$ value of $0.04;{mu}m/ml$ ($7.9{ imes}10^{-8};M$) and an inhibition constant ($K_i$) of $3.8{ imes}10^{-8};M$. This novel MR-387C was compared with various known inhibitors of aminopeptidase M. It inhibited the enzyme more strongly than any other microorganism-originated inhibitor, except probestin.