Tumor growth can increase the number of immature bone marrow-derived CD34+ cells that exhibit natural suppressor (NS) activity toward T-cell function. Using a metastatic Lewis lung carcinoma (LLC-LN7) tumor model, these CD34+ NS cells were shown to be present within the s.c. primary tumor tissue, but their levels declined after treatment with the inducer of myeloid cell differentiation, vitamin D3. Therefore, studies determined whether vitamin D3 treatment to diminish the CD34+ NS cell levels in LLC-LN7–bearing mice would enhance (a) intratumoral immune reactivity and (b) the antitumor activity of adoptive therapy consisting of tumor-reactive lymph node cells. The results showed that vitamin D3 treatment alone increased the intratumoral CD8+ cell content and the activity of the intratumoral infiltrate, as detected by production of interferon-γ and expression of the p55 IL-2 receptor. Although vitamin D3 treatment had no effect on the size of the primary tumor, it lessened the extent of tumor metastasis. Treating mice with the combination of vitamin D3 and adoptive immunotherapy significantly reduced metastasis in mice with established tumors, and reduced both metastasis and locoregional recurrence after surgical excision of the primary tumor. These studies demonstrate that vitamin D3 treatment increases intratumoral T-cell immune reactivity, and that coupling vitamin D3 treatment to diminish levels of CD34+ NS cells with adoptive immunotherapy enhances the effectiveness of the adoptively transferred tumor-reactive lymph node cells at limiting both metastasis and locoregional tumor recurrence.