To increase the yield of monoclonal antibody in a hybridoma culture, it is important to optimize the combination of several factors including cell density, antibody productivity per cell, and the duration of the culture. Potassium acetate enhances the production of antibodies by cells but sometimes depresses cell density. The production of anti-(human B-type red blood cell surface antigen) antibody by Cp9B hybridoma was studied. In batch cultures, potassium acetate inhibited Cp9B cells growth and decreased the maximal cell density but the productivity of antibody per cell was increased. The balance of the two effects resulted in a slight decline of antibody production. In a stirred tank bioreactor, the inhibitory effect of potassium acetate on cell density was overcome by applying the perfusion technique with the attachment of a cell-recycling apparatus to the bioreactor. In such a reactor, potassium acetate at 1 g l(-1) did not cause a decrease in the cell density, and the antibody concentration in the culture supernatant was increased from 28 μg ml(-1) to 38 μg ml(-1). Potassium acetate also suppressed the consumption of glucose and the accumulation of lactate in batch cultures, but the glucose and lactate levels were kept stable by applying the perfusion technique in the stirred tank bioreactor.