Because our earlier work indicated a strong correlation between specific antibody productivity and cell density in perfusion culture, we conducted experiments to determine the optimum means of increasing cell density while mainitaining high antibody productivity. The rates of medium supply and waste removal were varied to determine whether cell density was limited or inhibited, and whether a diffusable substance could be responsible for the correlation between antibody productivity and cell density. Nutrient supply was found to be a stronger determinant of cell density than waste removal; however, the rate of waste removal had a greater effect on cell growth at lower cell densities. Even at noninhibitory levels of ammonia and lactate, cellular metabolism was regulated to minimize their concentrations at lowered rates of waste removal. Separate step changes in glucose and glutamine resulted in increased cell density and antibody concentration. Specific antibody productivity increased following the step in glutamine, but not glucose. Both steps caused changes in cellular metabolism that prevented the levels of lactate and ammonia from reaching toxic levels.