Elevated pCO2 inhibits cell growth. This growth inhibition is accompanied by a decrease in intracellular pH (pHi), as well as a decrease in glycolysis. Elevated concentrations (mM) of some amino acids have been shown by others to protect cells exposed to two very different environmental stresses: nutrient starvation and hyperosmolality. The fact that many of the amino acids shown to have protective effects against other stresses are transported into the cell through a pHi-sensitive transporter led us to study the possibility of using these amino acids as protective agents under elevated pCO2. Screening experiments using 5, 15, and 25 mM of each amino acid showed that not all amino acids that protect cells from hyperosmolality protect them from elevated pCO2. Glycine betaine and glycine were chosen for further characterization in both hybridoma and CHO cells. Asparagine and threonine were also tested in hybridoma and CHO cells, respectively. All amino acids tested under 195 mm Hg pCO2/435 mOsm/kg (50% growth inhibition) restored the specific growth rate (μ) in hybridoma cells to that observed under control conditions (40 mm Hg/320 mOsm/kg). Addition of each amino acid resulted in an increase in the consumption rate and intracellular accumulation of that amino acid. In CHO cells, glycine betaine also restored μ to control values, while glycine and threonine partially restored μ. In hybridoma cells, the higher specific antibody productivity obtained at elevated pCO2 was maintained with the lowest amino acid concentration (5 mM). Productivity decreased toward control values with increasing amino acid concentrations. Elevated pCO2 decreased the specific tPA productivity in the CHO cell line studied. Only glycine betaine resulted in a 20% increase in productivity at 195 mm Hg/435 mOsm/kg. With the exception of glycine betaine in hybridoma cells, amino acids did not mitigate the associated pHi decrease of at least 0.2 pH units at 195 mm Hg/435 mOsm/kg. pHi in hybridoma cells under elevated pCO2 in the presence of glycine betaine was about 0.1 pH units below that of control. Amino acids had no effect on the cell size response of hybridoma cells, while they partially offset the increase in CHO cell size at elevated pCO2. Glycine betaine, asparagine, and glycine increased the specific glucose consumption rate observed at 195 mm Hg/435 mOsm/kg (50% of control) to values greater than 70% of control in hybridoma cells. In CHO cells, only glycine betaine increased qglc (by 20%) under elevated pCO2. All amino acids tested improved the cell yield from glutamine at 195 mm Hg/435 mOsm/kg in both cell lines. © 2002 Wiley Periodicals, Inc. Biotechnol Bioeng 78: 741–752, 2002.