The influence of aeration and glucose feeding on the stability of recombinant protein A in Escherichia coli during the transition period from a fed-batch cultivation to downstream processing was studied. Neither interruption of the feeding under aerobic conditions nor anaerobic conditions in presence of glucose could stabilize protein A completely and the intracellular ATP pool did not decrease to less than 0.75-1 mM by this treatment. On the other hand, the absence of both oxygen a nd glucose resulted in a decrease of the ATP pool to less than 0.5 mM and almost complete stabilization of protein A. The decrease of ATP was more severe when sulfite was used instead of nitrogen gas to create anaerobic conditions in presence of glucose. This also resulted in nearly complete stabilization of protein A, which might be explained by an inhibiting effect of sodium sulfite on fermentation. Therefore, protein stabilization and decrease of the ATP pool were correlated in experiments in vivo. The concentrations of ADP and AMP increased during starvation and may also play a role in stabilization of the protein in vivo. ATP may be a limiting factor of proteolysis also during further steps of downstream processing. Its concentration decreases by 80-90% during harvesting and centrifugation of biomass and even further during disruption of cells. However, neither addition nor regeneration of ATP in cell disintegrate was enough to restore degradation of protein A, indicating that an additional factor limits proteolysis in vitro.