Fed batch cultivations of plasmid-free and recombinant Escherichia coli were employed in order to determine cellular responses and effects of plasmid presence and induction on the host cell physiology. While plasmid presence was shown to have minor influence on overall biomass yield, induction with 0.1 mM IPTG led to a marked reduction. The number of dividing cells, measured as colony forming ability, was influenced by plasmid presence and to a larger extent by induction. The latter caused a decline in the number of dividing cells to less than 10% of the population within 10 h. However, this cell segregation did not affect the specific rate of product formation, which was approximately constant throughout the cultivations. Analysis of the in vivo degradation rate of the product indicated that it was proteolytically stable. The cellular content of the stringent response signal substance, ppGpp, peaked immediately after transition from batch to fed batch mode to stabilise at a higher value than in the batch phase. When the specific growth rate declined below 0.06 h-1 an additional rise in ppGpp concentration was observed.