Fed-batch cultures of recombinant Escherichia coli were studied regarding their ability to permit CI857(ts)-controlled gene expression at different growth phases. Cells, growing at 50% pO(2) and 28 degrees C in a bioreactor, were transferred to a shaker flask at 42 degrees C and the product yield was analysed after a short incubation time. Within the batch period, the synthesis of recombinant beta-galactosidase relative to the increase of biomass during the induction period was enhanced in an biomass-dependent fashion. Whereas in young cells biomass production dominates over recombinant protein production, the available energetic resources of aged eels seem to be employed more efficiently for CI857-controlled biosynthesis of recombinant proteins than of cell material. A model to describe this ’altruistic’ behaviour of old cultures is here proposed.