The sialic acid of glycoproteins secreted by recombinant Chinese hamster ovary (rCHO) cells can be impaired by sialidase under culture conditions which promote the extracellular accumulation of this enzyme. To investigate the effect of Bcl-x(L) overexpression on the sialylation of glycoproteins produced in rCHO cell culture, two rCHO cell lines producing the same Fc-fusion protein, which were derived from DUKX-B11 and DG44, respectively, were engineered to have regulated Bcl-x(L) overexpression using the Tet-off system. For both cell lines, Bcl-x(L) overexpression improved cell viability and extended culture longevity in batch cultures. As a result, a maximum Fc-fusion protein titer increased by Bcl-x(L) overexpression though the extent of titer enhancement differed between the two cell lines. With Bcl-x(L) overexpression, the sialylation of Fc-fusion protein, which was assessed by isoelectric focusing gel and sialic acid content analyses, decreased more slowly toward the end of batch cultures. This was because Bcl-x(L) overexpression delayed the extracellular accumulation of sialidase activity by reducing cell lysis during batch cultures. Taken together, Bcl-x(L) overexpression in rCHO cell culture increased Fc-fusion protein production and also reduced the impairment of sialylation of Fc-fusion protein by maintaining high viability during batch cultures. (c) 2015 American Institute of Chemical Engineers Biotechnol. Prog., 31:1133-1136, 2015