3,4-Dihydroxybenzoate decarboxylase in Enterobacter cloacae P241 was induced by adding 3,4-dihydroxybenzoic acid, 3-hydroxybenzoic acid, 3,4,5-trihydroxybenzoic acid or 4-acetamidobenzoic acid to the culture medium. After stabilizing the enzyme activity by adding 5 mM dithiothreitol and 20 mM Na(2)S(2)O(3) to a cell-free extract, catechol at 50 mM was carboxylated in the presence of 3 M KHCO(3) to 3,4-dihydroxybenzoic acid with a molar conversion ratio of 28% after 14 h at 30A degrees C.