MRK-22, an inhibitor of aminopeptidase M was isolated from the culture broth of Streptomyces sp. SL20209. The structure of MRK-22 was defined to be 3-amino-2-hydroxy-5-methylhexanoyl-valyl-valine, des-$Asp^4$-amastatin, by spectroscopic analysis and this was also confirmed by solid phase synthesis of the inhibitor. The molecular formula and weight of MRK-22 were $C_17H_33N_3O_5$ and MW 359($M^+$), respectively, and its $IC_50$ value against hog kidney AP-M was 0.79 $mu$ g/ml.