The ribosome and tRNA levels of Escherichia coli cells, transformed with a native or mutated Vitreoscilla hemoglobin genes (vhb), were investigated using asymmetrical flow field-flow fractionation (AFFFF). Mutagenesis of vhb by error-prone PCR was carried out to alter the growth behavior of microaerobically cultivated native VHb-expressing E. coli. A VHb mutant, pVMT1, was identified, which was able to reach a remarkably high final A(600) of 15, the value of which being 160% higher than that of a VHb control carrying pVHb8 (A(600) 5.8). AFFFF revealed that cells expressing mutant vhbs showed up to a doubling in the number of active 70S ribosomes cell(-1), an almost 3-fold increase in the number of tRNAs cell(-1), and up to a 26% increase in the mass fraction of active 70S ribosomes.