1. Starting from trans-cinnamic acid a chiral ( -)3-phenyl-[2,3-zH]propionic acid has been synthesized using Clostridium kluyveri cells as catalyst. 2.The chiral dideuterated acid has been converted by chemical methods to a mixture of (2R) and (2S)-phenyl[2,3-2H]-alanine.3. By means of 'H nuclear magnetic resonance spectroscopy and the action of D and L-amino-acid oxidase the configuration of the phenylalanine has been shown to be (2R, 3s) and (2S, 3S), respectively. The labelled phenylalanine is thus sterically and isotopically homogeneous at position 3 but heterogeneous at position 2.