Administration of Flt3 ligand (FL) to mice causes dendritic and natural killer cells to increase but certain solid tumors to regress. Depending on the particular tumor model used, T cells and natural killer cells have been implicated in the protective immune response induced by FL. The current study examined the effects of FL administration on tumor establishment and progression in metastatic and primary tumor models to correlate anatomic location with immunotherapeutic efficacy. FL mediated significant (p ≤ 0.05) therapeutic activity against pulmonary metastases of the murine MC-38 colon adenocarcinoma, particularly when cytokine administration was initiated before tumor inoculation. However, progressive intraabdominal tumors sometimes were observed even in the relative absence of pulmonary metastases. Significant, although less dramatic, antimetastatic effects were observed with MCA-205 and MCA-102 sarcomas and D5 (B16BL6) melanoma. In contrast, FL was ineffective against subcutaneous MC-38 tumors or against several intracranial tumors. This suggests that besides the administration dose, the efficacy of this cytokine depends on the tumor type and possibly the location of the inoculated tumor. Antitumor activities of FL were abolished by whole-body irradiation (500 cGy) and partially abolished by systemic depletion of CD8, CD4, or natural killer cells. The results indicate that optimization of FL immunotherapy of tumors will require a firmer understanding of the relative contributions of tumor burden, location, immune system requirements, and other factors.