Crystalline forms of EtO2C-Ch2-(R)Cgi-Aze-Pab-OH

There is provided EtO₂CCH₂(R)Cgl-Aze-Pab-OH, or a pharmaceutically-acceptable salt thereof, in a form which is substantially crystalline. It has been found that crystalline forms of EtO₂CCH₂(R)Cgl-Aze-Pab-OH have a high chemical and solid state stability when compared to amorphous forms of the compound.