The hybridoma cell line, HB-8696, produces a monoclonal antibody, 520C9 (mouse IgG(1)) that recognizes the breast cancer oncoprotein, c-erbB2. The effect of perfusion rate (volume of fresh feed/working volume of reactor/day) on cell growth and mAb production was investigated but perfusion at a constant rate and at an arbitrarily increased rate could not maintain exponential cell growth or a higher specific mAb production rate. An optimum step-up/step-down perfusion strategy is therefore proposed for maintaining a steady state production phase at high cell density for ten days. The optimum step-up perfusion could achieve fast cell growth by avoiding any nutrient limited condition and the following optimum step-down perfusion could potentially maintain high live cell density and reduced product dilution as well. The maximum viable cell achieved under optimum perfusion strategy was 2.3 x 10(7) cells/ml which was 19-fold higher than in optimum batch culture. The mAb yield and volumetric productivity were significantly improved to 52 and 50 mg/l day compared to 25 and 3.8 mg/l day in optimum batch, respectively, and could be maintained for up to ten days.