Interleukin-18 (IL-18) exhibits antitumor activity in various laboratory models. In the current study, brain tumors in naive mice regressed after an intratumoral injection of a single dose of recombinant IL-18 (rIL-18). Intraperitoneal rIL-18 substantially delayed the growth of subcutaneously inoculated gliomas but not gliomas located in the brain. Efficacy was reduced when studies were performed in mice depleted of natural killer cells. Although intracerebral administration of rIL-18 increased the serum interferon-gamma concentration, the antitumor effect of IL-18 was not mediated by interferon-gamma. These data suggest the therapeutic potential for control of tumor growth by intratumoral administration of rIL-18 in patients with glioma.