Chinese hamster ovary cells have been engineered to inducibly over-express the p21CIP1 cyclin-dependent kinase inhibitor, to achieve cell cycle arrest and increase cell productivity. In p21CIP1-arrested cells production of antibody from a stably integrated lgG4 gene, was enhanced approximately fourfold. The underlying physiological basis for enhanced productivity was investigated by measuring a range of cellular and metabolic parameters. Interestingly, the average cell volume of arrested cells was approximately fourfold greater than that of proliferating cells. This was accompanied by significant increases in mitochondrial mass, mitochondrial activity, and ribosomal protein S6 levels. Our results suggest that p21CIP1-induced cell cycle arrest uncouples cell growth from cell-cycle progression, and provides new insight into how improved productivity can be achieved in a cell line commonly used for large-scale production of pharmaceutical proteins. © 2004 Wiley Periodicals, Inc.