Some chemical elements have magnetic and nonmagnetic stable isotopes. The question arises if the magnetic fields of the atomic nuclei impact on efficiency of biocatalysts. Magnesium cation, Mg 2+ , serves as cofactor of ATP hydrolysis catalyzed by muscle myosin. Among three stable isotopes, 24 Mg, 25 Mg, 26 Mg with natural abundance 76.7, 10.13, 11.17 %, only 25 Mg is magnetic (nuclear spin I = 5/2) while 24 Mg and 26 Mg are nonmagnetic (I = 0). The effects of different isotopes of Mg on ATPase activity of isolated myosin were studied. It was found that the rate of the enzymatic ATP hydrolysis with 25 Mg is 2.0–2.5 times higher as compared to the nonmagnetic 24 Mg or 26 Mg. The magnetic isotope effect (MIE) was observed at physiological concentrations, 5 mM, of MgCl 2 . The similar MIE was found with zinc. Among five stable isotopes, 64 Zn, 66 Zn, 67 Zn, 68 Zn, and 70 Zn with natural abundance 48.6, 27.9, 4.1, 18.8, and 0.6 %, only 67 Zn is magnetic (I = 5/2). While Zn 2+ performs the cofactor function less efficiently than Mg 2+ , it was found that the rate of the enzymatic ATP hydrolysis with 67 Zn is 40-50 % higher as compared to the nonmagnetic 64 Zn or 68 Zn. On its own, MIE unambiguously testifies presence of a spin-selective rate-limiting step as the “bottle-neck” that is accelerated by the magnetic field of nuclear spin of 25 Mg or 67 Zn. In free-radical chemistry, MIE is the sufficient demonstration of a free-radical or radical ion pair as intermediate in the reaction under study (Buchachenko, 2013). One can assume that, under the condition of the electron-conformational excitation, there is a transfer of electron density from NH 2 group of Glu459 or OH– group of the bound water molecule to ADP3– or Mg 2+ / Zn 2+ in the active site of the enzyme with formation of the corresponding radical ion pair. Although detailed mechanisms of ability of myosin to perceive the nuclear magnetism require further investigations, the nuclear spin catalysis opens novel ways in free-radical biomedicine based on the stable magnetic isotopes. Supported by RFBR, project no. 14-04-00593.