The combined addition of reduced form of glutathione (GSH) and iron chelators for decrease of the intracellular level of reactive oxygen species (ROSS) and death of Chinese hamster ovary (CHO) cells was investigated. The addition of GSH to the serum-free cultures of CHO cells markedly lowered the intracellular ROS level, which effect was comparable to that of the addition of serum (0.4, 10%) and might be due to the reduction of ROS. The addition of iron chelators, deferoxamine and aurintricarboxylic acid, to the serum-free cultures also decreased the intracellular ROS level and increased the residual concentration of viable cells during the serum-free maintenance culture, which should be due to the inhibition of the Fenton and Harber-Weiss reactions stimulating the synthesis of ROS. The combined addition of GSH and the chelators decreased ROS level and cell death more effectively compared with their single addition.