The magnetic isotope effect (MIE) was discovered for isotopes of carbon as a relationship between the reaction rate (or probability of molecule formation) and nuclear spin, its projection, magnetic moment, and energy of electron nuclear (superfine) interaction [1]. MIE leads to isotope fractionation in dependence on their magnetic moment. This effect was later discovered [2] to occur for many pairs and triads of isotopes:Abstract-Lymphocytes from healthy donors and leukemic cells of patients with acute B lymphoblastic leu kemia (BALL 1) and acute myeloid leukemia were exposed to nanoparticles bearing magnetic (Zn 67) and nonmagnetic (total isotope pool) nuclei of zinc. The values of the corresponding magnetic isotope effects determined as the ratio of LD 50 magnitudes of preparations with magnetic and nonmagnetic zinc isotopes were 0, 3.5, and 1.5. Morphological studies using confocal and fluorescence microscopy showed apoptotic death of cells with the preparations; as well, there was an increase in the cell aggregation and better aggrega tion of nanoparticles in the case of 67 Zn NP, which resulted in a decrease of cytotoxicity. However the mag netic isotope effect was observed even in the case of aggregation.