Racemic 4-Chlorotryptophan was synthesized by an efficient route and the racemate was resolved by a chiral column into each enantiomer, the absolute stereochemistry of each was determined by reductive dehalogenation to the known chiral tryptophans. S-4-chlorotryptophan was then identified in the crude seed protein of Pisum sativum. 4-Chloroindole-3-acetic acid (4-Cl-IAA) and its methyl ester had been isolated in 1968 from immature seeds of pea, Pisum sativum, as a second natural auxin (plant hormone1,2,3,4) since the discovery of indole-3-acetic acid from human urine in 1934. Its possible biosynthetic precursor, 4-chlorotryptophan (1), had also been isolated in 1970 as the methyl malonamide derivatives (2) from the same immature seeds, exhibiting hypocotyl swelling activity when bioassayed with seedlings of Phaseolus spp. 4-Chlorotryptophan is the first naturally occurring unusual amino acid to be found. R=R1=H; 2a. R = Me, R1 = -CO-CH2-COOMe; 2b. R = Me, R1 = -CO-CH2-COOEt. Since pea seeds are one of the major protein sources taken by human beings, it is of extreme significance to examine whether this unusual amino acid is present in the protein fraction of pea seeds. In order to resolve this problem we have prepared chiral 4-chlorotryptophans (1a and 1b) (See Chart) as authentic samples for analysis, and achieved the identification of the S enantiomer (1b) in the crude seed protein of Pisum sativum, as described in this report.