The rate of DNA repair in human blood lymphocytes exposed to γ-radiation was shown to depend on the nuclear magnetic moment of magnesium ions. The decrease of DNA double-strand breaks occurs as a first-order reaction with the rate constants 1.1 × 10–3 s–1 and 1.6 × 10–3 s–1 for 25Mg2+ and 24Mg2+ ions respectively. Their ratio certifies magnetic isotope effect on the DNA repair; the latter occurs by 1.5 times slower with nuclear-magnetic 25Mg2+ ions than with nuclear nonmagnetic 24Mg2+ ions. The discovery of this effect is an irrefutable indicator that the DNA repair is spin-dependent and magneto-sensitive process, and its chemistry includes electron transfer and generation of radicals and radical pairs.