Magnetic isotope and magnetic field effects on the rate of DNA synthesis catalysed by polymerases β with isotopic ions 24 Mg 2+ , 25 Mg 2+ and 26 Mg 2+ in the catalytic sites were detected. No difference in enzymatic activity was found between polymerases β carrying 24 Mg 2+ and 26 Mg 2+ ions with spinless, non-magnetic nuclei 24 Mg and 26 Mg. However, 25 Mg 2+ ions with magnetic nucleus 25 Mg were shown to suppress enzymatic activity by two to three times with respect to the enzymatic activity of polymerases β with 24 Mg 2+ and 26 Mg 2+ ions. Such an isotopic dependence directly indicates that in the DNA synthesis magnetic mass-independent isotope effect functions. Similar effect is exhibited by polymerases β with Zn 2+ ions carrying magnetic 67 Zn and non-magnetic 64 Zn nuclei, respectively. A new, ion–radical mechanism of the DNA synthesis is suggested to explain these effects. Magnetic field dependence of the magnesium-catalysed DNA synthesis is in a perfect agreement with the proposed ion–radical mechanism. It is pointed out that the magnetic isotope and magnetic field effects may be used for medicinal purposes (trans-cranial magnetic treatment of cognitive deceases, cell proliferation, control of the cancer cells, etc).