Many chemical elements have magnetic and nonmagnetic stable isotopes. The question arises if living cells can perceive the difference. Of special interest is magnesium as one of the most abundant and obligate cell elements. Among three stable isotopes of Mg, 24Mg, 25Mg and 26Mg with natural abundance about 79, 10 and 11 %, only 25Mg has the nuclear spin (I = 5/2) and, hence, the nuclear magnetic field. Two other nuclei have no spin (I = 0) and no magnetic field. In the experiments with yeast S. cerevisiae, we revealed that the rate constant of the post-radiation recovery of cells after short-wave UV irradiation was twice higher for the cells enriched with 25Mg than the rate constant of the cells enriched with the nonmagnetic magnesium isotope . In the experiments with another commonly accepted cell model, E. coli, it was revealed that, upon transferring into novel growth media, the cells passed the adaptation period essentially faster if the growth medium was enriched with 25Mg in comparison with the media enriched with nonmagnetic 24Mg or 26Mg. In addition, the colony-forming ability of the cells grown on 25Mg has turned out to be essentially higher in comparison with the cells grown on the nonmagnetic isotopes . Thus, the magnetic isotope effects ("nuclear spin catalysis") in living cells have been discovered . Furthermore, the nuclear spin catalysis has been documented in the reaction driven by one of the most important “molecular motors” of cell bioenergetics, myosin. In the experiments with myosin isolated from uterus muscle, we revealed that 25Mg, by comparison to spinless 24Mg and 26Mg, doubles the rate of ATP hydrolysis catalyzed by this enzyme . These findings open the novel, based on stable magnetic isotopes, ways of control over efficiency and reliability of cell biomolecular nanoreactors. [Supported by RFBR, project no. 14-04-00593a].