Properties of purified L-tyrosine decarboxylase (EC 4.1.1.25) fromelicitor-induced cell suspension cultures of Eschscholtzia californicaCham. and Thalictrum rugosum Ait. are described. L-Tyrosine decarboxylaseis a dimeric enzyme with a molecular weight of 112,600 ± 600daltons. The isoelectric point was estimated to be at pH 5.2 and pH 5.4for the enzyme from E. californica and T. rugosum, respectively. Thepurified enzymes were stabilized in the presence of pyridoxal-5-phosphate.Optimum pH for the enzyme from both plants was found to be8.4. Enzyme activity was dependent on exogeneously supplied pyridoxal-5-phosphate. The enzyme decarboxylated L-tyrosine and L-,a-3,4-dihydroxyphenylalanine but was inactive toward L-phenylalanine and Ltryptophan.Apparent Km values of Eschscholtzia- and Thalictrum-decarboxylasefor L-tyrosine were 0.25 ± 0.03 and 0.27 ± 0.04 millimolar,respectively. Similar affinities were found for L-3,4-dihydroxyphenylalanine.Eschscholtzia L-tyrosine decarboxylase was strongly inhibitedby the phenylalanine analogue L-a-aminooxy-#-phenylpropionate andlargely unaffected by D,L-a-monofluoromethyl-3,4-dihydroxyphenylalanineand a-difluoromethyltyrosine.