A survey of the amino acids present in seeds of about forty species of the genus Acacia has revealed that members of the series Gummiferae can be distinguished readily from other species by their distinctive composition: among other differences, they alone contain N-acetyldjenkolic acid, together with small amounts of a related oxidized derivative, but they lack S-carboxyethyl- and S-carboxyisopropyl-cysteines, 3-acetylcqfl-diaminopropionic acid and albizziine, which are distributed widely among other Acacia species. L-fl-N-Acetyl-~,3-diaminopropionic acid has been characterized as a plant constituent for the first time, and evidence is presented that sulphoxide derivatives of S-carboxyethylcysteine, djenkolic acid and N-acetyldjenkolic acid coexist with the corresponding thioethers in many seeds of the Acacia genus.