A complex of polyethylene glycol conjugated with human recombinant cytokine interleukin-2 (IL-2) with antineoplastic activity. PEG-interleukin-2 induces natural killer (NK) cell activity and the production of interferon-gamma (IFN-gamma), and enhances T cell-mediated cytotoxicity. Pegylation of IL-2 protects the cytokine from degradation.