23.6 mg of L-valyl-L-prolyl-3-amino-2-oxo-pentanoyl-D-leucyl-L-valine prepared by the manner shown in Example 8 were dissolved in 2 ml of methanol. The resultant solution was admixed with a solution of 30 mg of 4-phenyl-butanoic acid anhydride in 1 ml of methylene chloride and with 0.050 ml of triethylamine, and stirred at room temperature for 1 hour. The reaction mixture was acidified with acetic acid, and subjected to a purification operation by employing a column (300 ml) filled with "Sefadex LH20 " with methanol, so that 20.9 mg of phenylbutanoyl-L-valyl-L-prolyl-3-amino-2-oxopentanoyl-D-leucyl-L-valine were obtained.
