a method for purifying an α - l - aspartyl - l - phenylalanine lower alkyl ester containing impurities . the α - l - aspartyl - l - phenylalanine lower alkyl ester is brought into contact with a cl - type anion exchange resin in an aqueous medium to have the impurities adsorbed by the anion exchange resin . the impurities contained in the α - l - aspartyl - l - phenylalanine lower ester , particularly diketopiperazine and α - l - aspartyl - l - phenylalanine are selectively and easily removed . the desired matter thus can be easily recovered and disposal of waste liquid , etc . can also be carried out without difficulty .