Porphyrinofullerene nanoparticles (NP) containing magnetic isotopes 25Mg and 67Zn (25Mg-NP and 67Zn-NP) and the natural isotopic composition of zinc (Zn) were tested on human leukemic cells of patients with acute leukemia and on lymphocytes of healthy donors. The fundamental differences in the cytotoxic effect of magnetic and nonmagnetic zinc isotopes on tumor cells were observed, as well as the complete absence of the influence of the magnetic magnesium isotope and pristine nanoparticles. The 67Zn-NP manifested high cytotoxicity towards cells of acute B-lymphoblast leukemia with LD50 almost three times lower than that of healthy donors and four times lower than that of the Zn-NP. Apoptosis was evaluated by cytofluorimetry for the drugs used.