Fourteen chemicals were used to treat Catharanthus roseus suspension cell cultures to improve ajmalicine, catharanthine or serpentine biosynthesis. Ajmalicine production was increased by betaine (to 55 mg l(-1)), n-propyl gallate (to 27 mg l(-1)), succinic acid (to 31 mg l(-1)), malic acid (to 60 mg l(-1)) and tetramethyl ammonium bromide (to 64 mg l(-)1). Ajmalicine and catharanthine yields were about 5-6 fold higher than the control. A large portion (up to 50-85%) of total indole alkaloids was released into the medium. For maximal catharanthine production, the optimal doses of malic acid and tetramethyl ammonium bromide were 50 mg l(-1) and 120 mg l(-1), respectively. The mechanisms which may be responsible for these treatment effects are discussed.