A membrane-bound γ-glutamyl transpeptidase is likely to be a key component of the 'amino acid site' of certain neutral amino acid transport systems. The hypothesis is in keeping with the Orlowski–Meister concept of a γ-glutamyl cycle for some forms of amino acid transport, and it is supported experimentally by a demonstration that the specificity of a γ-glutamyl transpeptidase (described herein) is virtually the same as the affinity of an amino acid site (described by Hajjar and Curran) for phenylalanine and 16 of its variously substituted analogues.Thus, both transpeptidase and transport site have a greater specificity for phenylalanine analogues having electron-withdrawing groups as substituents in the benzene ring than phenylalanine itself, and reduced specificity for analogues with electron-releasing substituents in the ring. The order of specificity according to ring substitution was −NO2 > −Cl > −F > −H > −CH3 > −OH > −NH2 for both systems. Again, the free α-amino group of phenylalanine plays a key role in the binding of the amino acid to the site, and it is also essential for the transpeptidase. Finally, the essential feature of the carboxyl group for binding of a neutral amino acid by the transport site is the −C = O group and this group is essential for maintaining the γ-glutamyl acceptor properties of an acceptor.