Effects of dissolved oxygen (DO) concentration on glutathione (GSH) production and cysteine oxidation were investigated in high cell density cultivation of Candida utilis. Lower DO concentration favors cysteine absorption but retards GSH production. Higher DO promotes GSH production but accelerates cysteine oxidation in the broth. A two-step DO control strategy was developed and compared for the potential in enhancing GSH production and cysteine absorption. By using the two-step DO control strategy, a 40% decrease in cysteine addition and a 13% increase in GSH production are observed as compared with that at constant DO of 40%. (c) 2007 Published by Elsevier Inc.