BACKGROUND: Hairy root cultures of Echinacea offer great potential for the production of valuable cichoric acid, but scale-up of the culture in the bioreactor represents a big challenge. Therefore, there is great interest in developing a suitable bioreactor for hairy root culture of Echinacea and novel bioprocessing strategies for intensifying cichoric acid production. RESULTS: Homogenous distribution of inoculum roots and high cichoric acid production were observed in a bioreactor modified by installing a mesh draught tube with an average pore size 700 gm, slightly larger than the hairy root, about 500 gm. Improved root growth and cichoric acid production were improved by increasing the aeration rate from 0.002 m(3) h(-1) to 0.012 m(3) h(-1). The hairy root cultures in the modified bioreactor exposed once to 6 min of ultrasound treatment at day 20 gave the highest biomass accumulation of 12.8 +/- 0.3 g L-1, which resulted in the maximum cichoric acid production of 178.2 +/- 4.9 mg L-1 at day 30. CONCLUSION: The present work demonstrated the effectiveness of hairy root culture in a modified airlift bioreactor. The biomass distribution remained homogenous in the modified airlift bioreactor, and the cichoric acid production was improved owing to the even root growth at optimal air flow rate. An interesting finding of this investigation was that ultrasound stimulated root growth and cichoric acid production considerably in the modified airlift bioreactor. (C) 2009 Society of Chemical Industry