BACKGROUND: In this work recombinant gp-41 protein production by E. coli in an unconventional bioreactor coupled with a magnetic field generator was studied. The submerged fermentation process was carried out by fed-batch operation, recycling the culture medium externally through a stainless steel spiral U-shape tube inserted on the inside of a magnetic field generator. The exposure time and magnetic field induction were varied in a range from 1 to 12 h and 10 to 100 mT, respectively, according to a central composite design with centered face. RESULTS: It was found that the inhibitory or stimulatory effect of magnetic field on cell growth and protein production depended on exposure time and magnetic field strength. The production of recombinant gp41 protein was 20% higher than the control. CONCLUSION: The procedure here presented could be an easy-to-use approach to improve the efficiency of recombinant protein production by E. coli. (c) 2007 Society of Chemical Industry.