Genetic manipulation of the host strain, by which cell physiology could be modulated, was exploited to enhance recombinant protein production in Escherichia coli. The effects of an inactivated stationary-phase gene (rmf or katF) on recombinant protein production in strains with two different expression systems (the pH-inducible and the lac promoters) were investigated. An improvement of recombinant protein production in the katf muta nt at low growth rates was observed for both expression systems. A fourfold and a 30% increase in the volumetric recombinant protein activity were observed for the pH-inducible and the lac promoter system, respectively. The effect of the rmf mutation, on the other hand, depends on the expression system. A twofold increase in the volumetric recombinant protein activity was found for the pH-inducible promoter system, but there was no improvement for the lac promoter system. Improvement in culture performance for slow-growing cultures may have an impact on the design strategy of the host/vector system used in fed-batch cultures, where the specific growth rate is usually slow. The information may also be useful for developing optimal host/vector gene expression systems for recombinant protein production.