The rate of ATP synthesis by creatine kinase extracted from V. xanthia venom was shown to depend on the magnetic field. The yield of ATP produced by enzymes with Mg-24(2+) and Mg-26(2+) ions in catalytic sites increases by 7-8% at 55 mT and then decreases at 80 mT. For enzyme with Mg-25(2+) ion in a catalytic site, the ATP yield increases by 50% and 70% in the fields 55 and 80 mT, respectively. In the Earth field the rate of ATP synthesis by enzyme, in which Mg2+ ion has magnetic nucleus Mg-25, is 2.5 times higher than that by enzymes, in which Mg2+ ion has nonmagnetic, spinless nuclei Mg-24 or Mg-26. Both magnetic field effect and magnetic isotope effect demonstrate that the ATP synthesis is an ion-radical process, affected by Zeeman interaction and hyperfine coupling in the intermediate ion-radical pair.