The effects of low-frequency magnetic fields (Bm=2.7-10 mT, f=50 Hz, time of exposure t=0-12 min, laboratory temperature) on the viability and oxidoreductive activity of gram-negative bacteria Escherichia coli were investigated. The growth of these bacteria was negatively affected by such fields. We compared two experimental systems--solenoid [Sb. Lek. 99 (1998) 455] and a cylindrical spool--to find differences between nonhomogeneous and "more homogeneous" magnetic fields. We observed analogous effects in both experimental conditions. The growth curve of the exposed bacteria was lower than the control one. The ability of bacteria to form colonies decreased with increasing magnetic field intensity and with increasing time of exposure. The oxidoreductive activity was measured using reduction of a tetrazolium salt. The decrease in oxidoreductive activity with increasing time of exposure was observed, but the effect was due to a lower amount of bacteria surviving the exposure to the magnetic fields. The decrease in oxidoreductive activity and ability to form colonies were compared with the assumption that the effect of magnetic field is probably bactericidal.