Fermentation with complete cell recycle on microfiltration tubular membranes was studied and its effect upon broth rheology evaluated. The increase in cell concentration was found to be responsible for a continuous rise in viscosity leading to changes of broth rheology and consequent operating problems. A strategy to control the broth rheological behaviour with the aim of achieving long-term stable fermentations is proposed. This strategy involves the use of a cell purge in order to maintain cell concentration at a manageable level. The results obtained show that the total cell concentration in the fermenter can be stabilised if the cell bleed rate is calculated to remove exactly the amount of cells produced. To obtain long-term stable fermentations, the time to initiate the bleeding procedure and the cell bleed rate need to be determined and optimised.