Enhanced product yields, reduction in throughput time, improved cost-effectiveness and product quality are examples of benefits gained by delaying apoptotic cell death in bioreactors. To examine the effect on recombinant protein production, bcl-xL was overexpressed in a CHO cell line secreting humanized monoclonal antibody directed against the α1β1 integrin. When cell lines overexpressing bcl-xL were compared to the parent, cell viability was increased by 20% and titers by 80%. Total viable cell densities were similar and specific productivities were enhanced by almost two-fold on scale-up to bioreactors. Comparison in a chemically defined media demonstrated an even greater sustained viability in bcl-xL expressing cells by 50% and up to 90% increase in titer with no impact on product quality. Caspase 3 activities were monitored as a marker for apoptotic cell death. In the presence of Bcl-xL, caspase activities were reduced to background levels. The role of Bcl-xL in protecting cells from premature death was further examined in studies performed in the presence of NaBu, at concentrations known to trigger cell death. Results demonstrated that cells expressing bcl-xL retained 88% cell viability with >2 fold increase in titer. Bcl-xL was similarly overexpressed in a different CHO cell line producing a humanized mAb against the chemokine MCP1. Once again, production titer was increased by 80% and viability by 75%. Together the studies have shown that overexpression of bcl-xL in production cell lines was able to significantly increase the titer by enhancing both the specific activity and total cell viability while maintaining product quality. © 2005 Wiley Periodicals, Inc.