In 250 ml of tetrahydrofuran are dissolved 23.7 g (80.4 millimoles) of Z-Ser(But)-OH and 10.2 g of N-hydroxysuccinimide, and the solution is cooled to 0° C. and 16.6 g of dicyclohexylcarbodiimide is added to the solution. The mixture is stirred for 3 hours at the above temperature and then stirred in a low temperature chamber overnight. The formed precipitate is removed by filtration and the filtrate is concentrated under reduced pressure. The obtained oily substance is dissolved in 100 ml of dimethylformamide. This solution is mixed with a solution formed by dissolving 35.5 g (80.4 millimoles) of H-Arg(Tos)-Leu-OH prepared according to the process (ii) of Example 8 in 200 ml of dimethylformamide and adding 11.2 ml of triethylamine to the solution, and the mixture is stirred for 6 hours at 6° C., overnight in a low temperature chamber and for 5 hours at room temperature. The insoluble substances are removed by filtration and the filtrate is concentrated under reduced pressure. The residue is extracted with 700 ml of ethyl acetate, washed with 2% citric acid and water and dried with magnesium sulfate. The dried filtrate is concentrated under reduced pressure and ether is added to the residue to effect solidification. The solid is dissolved in a mixed liquid of chloroform and methanol, and the solution is concentrated under reduced pressure. Solidification is effected by ethyl acetate to obtain 41.9 g of intended Z-Ser(But)-Arg(Tos)-Leu-OH (the yield being 73%).
