Feeding of cinnamic acid and ferulic acid to non-treated andchitosan-treated cell suspension cultures of Vanilla planifoIiaresulted in the formation of trace amounts of p-hydroxy benzoicacid (5.2 micrograms per gram fresh weight of cells) and vanillicacid (6.4 micrograms per gram fresh weight of cells), respectively.Addition of a 4-hydroxycinnamate: CoA-ligase inhibitor, 3,4-(methylenedioxy)-cinnamic acid (MDCA), resulted in a reducedbiosynthesis of ligneous material with a simultaneous significantincreased vanillic acid formation (around 75 micrograms per gramfresh weight of cells). A K, of 100 micromolar for 4-hydroxycinnamate:CoA-ligase in a crude preparation was estimated for thisinhibitor. It is suggested that the conversion of cinnamic acidsinto benzoic acids does not involve cinnamoyl CoA esters asintermediates. Feeding of 14C-cinnamic acid and 14C-ferulic acidto cells treated with MDCA indicate that cinnamic acid, but notferulic acid, is a precursor of vanillic acid in these cultivated cellsof V. planifolia.