Cyclo(Δ-Ala-L-Val) is a diketopiperazine formed by the fusion of Δ-alanine and valine, reported as a secondary metabolite of fungi and bacteria. In Pseudomonas aeruginosa, cyclo(Δ-Ala-L-Val) is capable of activating N-acylhomoserine lactones (AHLs). Cyclo(Δ-Ala-L-Val) is also capable of activating or antagonizing other LuxR-based quorum-sensing systems. While the mode of action of cyclo(Δ-Ala-L-Val) is not known, its activity suggests the existence of cross talk among bacterial signalling systems.