The effect of the extremely low frequency (ELF) magnetic field on nisin production by Lactococcus lactis subsp. lactis using cheese whey permeate was studied during batch fermentation. The cellular suspension from the fermentor was externally recycled through a stainless steel tube inserted between the magnetic bars. The exposure time, recycle velocity and intensity of the magnetic field varied in a range of 4-12 h, 0.85-1.50 m s(-1) and 5-20 mT, respectively, according to a 2(3) Complete factorial design. In all experiments, the produced biomass was almost constant. However, under the best conditions with the magnetic field treatment (4 h, 1.50 m s(-1) and 5 mT) the nisin yield with respect to the substrate consumption and formed biomass were three and five times higher than the values of the control experiment. The application of the magnetic field allowed to deviate the metabolic pathway in order to intensify or to inhibit the nisin production. (c) 2006 Elsevier Ltd. All rights reserved.