Abstract: A process for isolating substantially pure natural L-.beta.-3-indolylalanine (L-.beta.-3) from a mixture of amino acids, such as a protein hydrolysate, is described. A protein hydrolysate, for example of casein or soy protein, is passed over a polymeric resin attractive to aromatic amino acids but not attractive to aliphatic amino acids. The aromatic amino acids are retained on the resin while the aliphatic amino acids pass over the resin and are collected. The resin is then washed with water to displace any residual aliphatic acids which may be physically associated with but not bound to the resin. Thereafter, the resin is eluted with a dilute acid to displace L-phenylalanine and L-tyrosine and provide a solution thereof while allowing L-.beta.-3 to be retained on the resin. The resin is then further eluted with a dilute base to displace L-.beta.-3 from the resin and provide a solution of L-.beta.-3. Substantially pure natural L-.beta.-3 is recoverable from this solution.