This study investigates the antitumor and antimetastatic effects of murine interleukin 12 (IL-12) against various murine tumors. IL-12 administration to mice resulted in enhanced cytotoxic natural killer (NK) cell and cytolytic T cell activity, as well as increased interferon γ (IFN-γ) secretion. The study found that IL-12 significantly reduced experimental pulmonary metastases and subcutaneous growth of the B16F10 melanoma, with a dose-dependent increase in survival time. IL-12 was also effective against established hepatic metastases and subcutaneous M5076 reticulum cell sarcoma and Renca renal cell adenocarcinoma tumors, without causing gross toxicity. Local peritumoral injections of IL-12 into established subcutaneous Renca tumors led to regression and complete disappearance of these tumors. The antitumor efficacy of IL-12 was mediated primarily through CD8+ T cells, as demonstrated by the reduced efficacy in nude mice and the depletion of CD8+ T cells. These findings highlight the potent antitumor and antimetastatic properties of IL-12 and the critical role of CD8+ T cells in mediating its effects.This study investigates the antitumor and antimetastatic effects of murine interleukin 12 (IL-12) against various murine tumors. IL-12 administration to mice resulted in enhanced cytotoxic natural killer (NK) cell and cytolytic T cell activity, as well as increased interferon γ (IFN-γ) secretion. The study found that IL-12 significantly reduced experimental pulmonary metastases and subcutaneous growth of the B16F10 melanoma, with a dose-dependent increase in survival time. IL-12 was also effective against established hepatic metastases and subcutaneous M5076 reticulum cell sarcoma and Renca renal cell adenocarcinoma tumors, without causing gross toxicity. Local peritumoral injections of IL-12 into established subcutaneous Renca tumors led to regression and complete disappearance of these tumors. The antitumor efficacy of IL-12 was mediated primarily through CD8+ T cells, as demonstrated by the reduced efficacy in nude mice and the depletion of CD8+ T cells. These findings highlight the potent antitumor and antimetastatic properties of IL-12 and the critical role of CD8+ T cells in mediating its effects.