The effects of ammonia on the growth of recombinant CHO cells and the expression of EPO were studied. The inhibition of ammonia over the growth of recombinant CHO cells follows the second order inhibition model, and the inhibition constant K a is 41.5(mmol/L) 2, i.e. the specific growth rate of cells reduced to the half of its maximum value at 6.45 mmol/L ammonia. It was shown that the cell density of recombinant CHO cells and EPO concentration in batch cultures with different initial ammonia concentrations reduced with the increase of ammonia concentrations. The specific production rate of EPO at 3.22 mmol/L ammonia was higher than those at others, showing that there was an optimum ammonia concentration at which the capacity of recombinant CHO cells expressing EPO was on the peak. These results may be helpful to optimize the production of EPO in the culture of recombinant CHO cells.