A recombinant peptide similar to or identical to endogenous human cytokine interleukin-21 (IL-21) with potential antineoplastic activity. Denenicokin binds to and activates IL-21 receptors, expressed on T-cells, B-cells, dendritic cells (DC), and natural killer (NK) cells, modulating the proliferation and/or differentiation of T and B cells, promoting T cell survival, and increasing the cytolytic activity of cytotoxic T lymphocytes (CTLs) and NK cells.