We evaluated the impact and mechanism of interleukin (IL)-18 alone or in combination with IL-12 or tumor necrosis factor-alpha when delivered intratumorally via polylactic acid microspheres (PLAMs). C57BL6 mice with established B16 melanomas underwent a single intratumoral injection of IL-12, tumor necrosis factor-alpha, or IL-18 PLAM, alone or in combination. Tumor draining lymph nodes and splenocytes were assessed for specific antitumor response by FACS analysis and IFN-gamma release assay and enzyme-linked immunosorbent spot. Mice with established pulmonary metastases were killed for enumeration of pulmonary metastatic nodules after treatment of the primary tumor. Intratumoral treatment with IL-12 in combination with IL-18 led to significant tumor suppression compared with either cytokine alone. FACS analysis revealed the combination of IL-12 and IL-18 resulted in an increase in the percentage of CD3+ cells within the tumor draining lymph node, attributable to increases in both CD4+ and CD8+ T cells. Both IFN-gamma release assay and enzyme-linked immunosorbent spot demonstrated a significant and substantial increase in tumor-specific response with the combination. Treatment of the primary tumor with IL-12 and IL-18 PLAM led to a significant decrease in pulmonary metastases and improvement in survival compared with either cytokine alone. The systemic effects were abrogated after depletion of CD8+ or natural killer cells, but not CD4+ cells. IL-12 and IL-18, when released intratumorally in a sustained fashion as can be accomplished through the use of PLAM, demonstrate both local effects on tumor growth and the generation of a tumor-specific response capable of eradicating distant disease.