Our previous study demonstrated that manganese oxide nanoparticles (MnO NP) selectively destroyed U-87MG and U251 human glioblastoma cells in vitro. MnO NP were synthesized and studied by electron microscopy. Their antitumor properties were studied in vivo on the model of immunodeficient SCID mice with subcutaneous xenografts of U-87MG human glioblastoma. The mice were injected subcutaneously with MnO NP in doses of 0.96 and 1.92 mg/kg (calculated for Mn) 3 days a week over 3 weeks. In was shown that MnO NP in these doses significantly suppressed the growth of U-87MG glioblastoma xenografts: on day 21 from the start of the treatment, the tumor growth inhibition index was 61.1 and 99.22%, respectively. These results indicate the necessity of the further studies of MnO NP as a potential oncolytic agent for the therapy of human glioblastomas.