Tyrosine is proved to be decarboxylated with retention of configuration by three aromatic amino-acid decarboxylases of bacterial, mammalian, and plant origin by experiments based on the synthesis of (αS)-[α-³H]tyrosine and the (αR)- and (αS)-isomers of [α-³H₁]tyramine. Incubation of the labelled tyramines with the diamine oxidase from pea seedlings and the monoamine oxidase from rat liver shows that the former enzyme removes H_si and the latter H_Re. These amine oxidases are used to assay the chirality of the [α-³H₁]tyramines produced by incubation of suitably labelled tyrosines in tritiated water or in unlabelled water with the partially purified decarboxylases from Streptococcus faecalis and hog kidney. The plant enzyme was studied by administration of (αS)-[α-³H,U-¹⁴C]tyrosine to intact Papaver somniferum plants followed by isolation of the resultant papaverine, which retained no tritium; morphine was also isolated to act as an internal standard.