The study of the antineoplastic activity of the nano-preparation “Selenium” applied in various doses in 40 non-purebred mice with the intertwined intra-abdominal-ascite carcinoma of Ehrlich is carried out. The ascite liquid volume, the tumor cell quantity in 1 ml, the life duration, the extent of tumor growth inhibition and the increase in life duration at the intra-abdominal introduction of the nanocomposite “Selenium” in the single introduction in 24 hours after a re-inoculation were assessed. The effects were compared to the ascite carcinoma of Ehrlich. The antineoplastic action of the nano-preparation “Selenium” was noted. The perspective of the further research of the nanocomposite “Selenium” for the purpose of application as the antineoplastic preparation is proved.