The CHO320 cell line, engineered to produce human interferon gamma was in investigated with regard to its susceptibility to oxidative stress. Batch cultures of the cells grown in a bench-top bioreactor exhibited no marked response to changes in oxygen concentration between 6% and 14% whereas cell growth and recombinant protein production were inhibited by increasing the oxygen to 20%. High concentrations of hydrogen peroxide (in excess of 200 mu M) were required to inhibit growth of the CHO320 cells whereas concentrations of 50 mu M and 100 CIM had no effect on recombinant protein production. Buthionine sulphoximine (50 mu M and 100 mu M) completely depleted the cells of glutathione within 24 h; however, no quantitative effect on recombinant protein production was seen. It is concluded that the CHO320 cells are, possibly as a consequence of the long selection process they have undergone, very resistant to oxidative stress.