Tyrosine decarboxylase (EC 4.1.1.25) from Syringa vulgaris L. cell cultures and from Hordeum vulgare L. seedlings is strongly inhibited by the phenylalanine analogue, L-α-aminooxy-β-phenylpropionate. L-α-Aminooxy-β-phenylpropionate is therefore not specific in its inhibitory action against