A recombinant therapeutic agent which is chemically identical to or similar to the endogenous cytokine interleukin-18 (IL-18). Produced primarily by macrophages, IL-18 induces the production of interferon-gamma (IFN-gamma), and enhances the activity of natural killer (NK) and cytotoxic T lymphocytes (CTL). As a potential immunotherapeutic agent, IL-18 displays antitumor effects in vitro and in animal models.