To explore the influence of ultra-low carbon dioxide partial pressure (pCO(2)) on the monoclonal antibody (mAb) N-glycosylation profile in Chinese hamster ovary (CHO) cell culture. In fed-batch bioreactor cultures, lowering the pCO(2) in the medium (< 25 mmHg) via increasing headspace aeration decreased the cell viability and mAb production in CHO cells. Additionally, mAb galactosylation under low pCO(2) was approximately 27.45 +/- 2.13%, noticeably higher than that observed under normal pCO(2) (21.36 +/- 1.66%) at harvest. However, all of the relevant intracellular nucleotide sugar concentrations were dramatically decreased to approximately 50% of the levels found under normal pCO(2) on day 7. Real-time PCR revealed that the upregulation of galactosylation-related glycosyltransferase genes and substrate transporter genes played a critical role in the improved galactosylation under the ultra-low pCO(2) condition. In the bioreactor culture processes, ultra-low pCO(2) demonstrated a positive effect on mAb galactosylation.