Animal cell (Chinese Hamster Ovary) concentration was determined on-line in a packed bed process using dielectric spectroscopy. This enabled the evaluation of the effect of temperature on specific metabolic rates during 3 months of continuous culture. The effect of low cultivation temperature on cell growth and metabolism was monitored, and the data were used for process development. At 37degreesC cells grew exponentially with a specific growth rate of 0.038 d(-1) and specific glucose uptake and lactate production rates increased continually. Reduction of the temperature to 33.5degreesC resulted in a lowering of these metabolic rates while having no effect on cell proliferation. Subsequent reduction of the temperature to 32degreesC resulted in stabilization of the cell concentration at a high density (3.6 x 10(7) cell per mL of packed bed). In addition, the specific production rate of the protein of interest increased by a factor of 6 compared to the value at 37degreesC. During the stationary phase at 32degreesC, all other specific metabolic rates could be controlled to low and constant levels.