γ-L-Glutamyl-L-α-(methylenecyclopropyl)glycine has been characterized as a new plant constituent from seeds of Billia hippocastanum and fruits of Acer pseudoplatanus. The sycamore fruits also contained hypoglycins A and B, and α-(methylenecyclopropyl)glycine. The γ-glutamyl peptides of aspartic acid, asparagine, glutamic acid, valine, alanine and threonine were also isolated from Billia seed.