Different feeding strategies for the production of human interferon-gamma using an isopropyl beta-D-thiogalactoside-inducible expression system in recombinant Escherichia coli BL21(DE3) (plasmid pET3a-ifngamma) were studied. Four fed-batch modes were designed to compare the effect of mu (specific growth rate) on recombinant-protein production, substrate consumption, by-product formation and plasmid stability during pre- and post-chemical induction in high-cell-density cultures of E. coli. It was found that Y(p/s), the product/substrate yield of interferon-gamma was significantly affected by mu throughout the process, but product/biomass yield (Y(p/x)) was influenced by mu at the pre-induction stage. By applying an efficient feeding strategy, in which the mu was maintained at the maximum attainable level, recombinant protein was accumulated up to a level of 60% of the total cell protein and its productivity was increased significantly. In this case, the overall productivities of biomass and recombinant protein were 6.36 g l(-1) h(-1) and 2.1 g l(-1) h(-1) respectively, in comparison with 1.91 g l(-1) h(-1) and 0.16 g l(-1) h(-1) during exponential feeding, in which the specific growth rate was kept constant throughout the entire process.