The studies on the biosynthetic precursors of the modified amino acids in syringomycin and syringopeptin, lipodepsipeptides from the phytopathogenic bacterium Pseudomonas syringae pv. syringae were carried out by feeding 14C labelled precursors to the bacterial cultures, followed by the isolation of the labelled metabolites, degradation and derivatization of the single residues. Thr was shown to be the precursor of dehydrothreonine and, surprisingly of 4-chlorothreonine.