The inhibition of enzymatic activity of the ATP-synthesizing creatine kinase by CH3HgCl is shown to strongly depend on the mercury isotope substitution. The efficiency of inhibition does not depend on the nuclear mass of the mercury isotopes; however, it is different for magnetic (Hg-199, Hg-201) and nonmagnetic (Hg-200, Hg-202) nuclei. When mercury isotopes in CH3HgCl are presented in natural abundance, the reaction of creatine kinase with CH3HgCl fractionates magnetic and nonmagnetic mercury isotopes. These observations demonstrate that the reaction between creatine kinase and CH3HgCl is nuclear spin selective; that is, isotope fractionation is induced by the magnetic isotope effect. According to the suggested reaction scheme, it operates in the ion-radical pairs generated in enzyme active site from CH3HgCl and cysteine residue as the reaction partners.