A prolonged period of high productivity at high cell density is desirable for industrial production of biopharmaceuticals. Previous efforts have shown that cessation of cell proliferation in low cell density culture results in increased productivity. a report here further results on multigenic manipulation of cell cycle and apoptosis to enhance productivity at high cell density. The NS0 6A1/4-9F myeloma cell line, which constitutively expresses a. chimeric IgG4 antibody and inducibly expresses the p21(CIP1) cyclin-dependent kinase inhibitor has been further engineered to constitutively overexpress the Y28 mutant Bel-anti-apoptotic protein. The effects of overexpression of p21(CIP1) and Bcl-2 on cell proliferation, cell viability, and antibody production has been investigated in batch and continuous perfusion cultures. In both cultures the p21(CIP1) protein arrested cell proliferation, confirming the previous results in low-density culture of 4-fold. increase in antibody production, whereas mutant Bcl-expression has not resulted in any significant improvement in cell viability of arrested cells. This study demonstrates that it is possible to enhance the productivity of relatively high-density continuous mammalian cell cultures by arresting the cell cycle in G1 phase.