Nanocomposition comprised of interleukin-2 in suboptimal noneffective concentration and β-cyclodextrin was studied in vitro. This preparation as well as interleukin-2 in optimal concentration was shown to increase natural killer activity to K-562 cells and cytotoxicity of activated peripheral blood mononuclear cells (PBMCs) against PC-3 and DU 145 cells. At the same time β-cyclodextrin or interleukin-2 in equimolar concentrations did not influence the spontaneous killer activity of PBMC. This combination of cyclodextrin + interleukin-2 led to the decrease of interleukin-2 effective concentration by an order. This phenomenon could be explained by cyclodextrins ability to promote the formation of nanoparticles with drugs, which results in enhancing their water solubility and bioavailability. Besides, interleukine-2/β-cyclodextrin nanocomposition as opposed to interleukin-2 alone led to increasing the number of not only lymphocytes, but also macrophages contained in activated PBMC population. Application of low concentration of interleukin-2 allowing for good clinical efficiency may significantly mitigate the side effects of the drug and enable to develop adoption of immunotherapy for patients with androgen-resistant prostate cancer.