This work studies biological effects of low-frequency electromagnetic fields. We have exposed three different bacterial strains - Escherichia coli, Leclercia adecarboxylata and Staphylococcus aureus to the magnetic field (t < 30 min, Bm=10 mT, f = 50 Hz) in order to compare their viability (number of colony-forming units (CFU)). We have measured the dependence of CFU on time of exposure and on the value of the magnetic field induction Bm. Viability decreases with longer exposure time and/or higher induction Bm for all strains, but the quantity of the effect is strain-dependent. The highest decrease of the viability and the biggest magnetic field effect was observed with E. coli. The smallest magnetic field effect appears for S. aureus. From the measurement of the growth dynamics we have concluded that the decrease of the CFU starts immediately after the magnetic field was switched on.