The productivity of a cell culture for the production of a secondary metabolite is defined by three factors: specific growth rate, specific product formation rate, and biomass concentration during production. The effect of scaling-up from shake flask to bioreactor on growth and production and the effect of increasing the biomass concentration were investigated for the production of ajmalicine by Catharanthus roseus cell suspensions. Growth of biomass was not affected by the type of culture vessel. Growth, carbohydrate storage, glucose and oxygen consumption, and the carbon dioxide production could be predicted rather well by a structured model with the internal phosphate and the external glucose concentration as the controlling factors. The production of ajmalicine on production medium in a shake flask was not reproduced in a bioreactor. The production could be restored by creating a gas regime in the bioreactor comparable to that in a shake flask. Increasing the biomass concentration both in a shake flask and in a stirred fermenter decreased the ajmalicine production rate. This effect could be removed partly by controlling the oxygen concentration in the more dense culture at 85% air saturation.@en