The addition of 1-aminocyclopropane-1-carboxylic acid (ethylene precursor), or 2-chloroethylphosphonic acid (ethephon, an ethylene-releasing compound) decreased root dry weight and L-DOPA (L-3,4-dihydroxyphenylalanine) accumulation in hairy root cultures of Stizolobium hassjoo. The inhibition caused by ethephon-mediated ethylene release was alleviated by 0.5 mg CoCl2 l(-1) as an inhibitor of ethylene biosynthesis. The action of ethylene was inhibited by 1.5 mg AgNO3 l(-1). Ethylene thus lowers hairy root formation and L-DOPA production; CoCl2 decreases ethylene formation leading to a considerably improved root dry weight and L-DOPA production.