L-Pyroglutamic acid (pidolic acid, 5-oxo-L-proline , (S)-5-oxopyrrolidine-2-carboxylic acid) is a naturally occurring γ-lactam formed enzymically. It may also be formed synthetically by the thermal head-to-tail cyclisation of L-glutamic acid. Elevated plasma levels of pyroglutamic acid are associated with aberrant glutamine and glutathione metabolism. L-Pyroglutamic acid is used in dietary supplements for cognitive and memory enhancement, although such use is not clinically validated. Microbial metabolites consisting of pyroglutamide conjugated with amino acid esters have been isolated from Lactobacillus plantarum and possess immunomodulatory activity. Indeed, the thioproline-conjugated amide, pidotimod, is a commercially available immunostimulant.