By using polymerases beta loaded with isotopic ions Mg-24(2+), Mg-25(2+) and Mg-26(2+) magnetic isotope effect was detected: Mg-25(2+) ions with magnetic nuclei Mg-25 suppress enzymatic activity by 2-3 times with respect to that of polymerases beta loaded by Mg-24(2+) and Mg-26(2+) ions. No difference in enzymatic activity of polymerases beta with Mg-24(2+) and Mg-26(2+) ions exists. The rate of DNA synthesis strongly depends on the magnetic field. The polymerase chain reaction is also suppressed by Mg-25(2+) ions with respect to the ions with nonmagnetic nuclei. Magnetic control of the DNA synthesis may be used for medical purposes. (C) 2013 Elsevier B. V. All rights reserved.