In reduced glutathione (GSH) production by high-cell-density fed-batch cultivation of Saccharomyces cerevisiae T65, a respiratory quotient (RQ) feedback control strategy was applied to determine glucose feeding rate (F) based anon-line off-gas monitoring in a 15-l stirred bioreactor. Glucose feed was manipulated by a classical proportional-integral-derivative (PID) controller to control RQ at its set-point. RQ controlled at 0.65 resulted in the highest GSH productivity (46.9 mg/l/h) and cell productivity (3.5 g/l/h) as well as improved GSH yield and intracellular GSH content. RQ feedback control achieved yeast dry weight (126 g/l), GSH yield (2.1 g/l), GSH content (1.67%) and GSH productivity (55.3 mg/l/h). which improved by 11.5%, 75%, 57.5% and 82.5%, respectively, compared with conventional ethanol feedback control in the GSH industry. Moreover. RQ feedback control reduced ethanol (byproduct) level to below 0.3 g/l. Advantages of RQ feedback control over the conventional ethanol feedback control in the GSH industry include lower byproduct concentration, shorter cultivation period, higher GSH content, and higher GSH yield. Hence, feedback control based on RQ is a promising method that poses as a simple and efficient alternative to conventional feed control techniques presently practiced in the GSH industry. (C) 2010 Published by Elsevier Inc.