The role of natural killer T (NKT) cells in the immune response to tumor cells has been largely unexplored. As a model of adoptive tumor immunotherapy, cells from the draining lymph nodes of mice immunized with a tumor-specific or irrelevant antigen were transferred to naïve recipients with established tumor. Inhibition of early tumor growth (day 4) required the transfer of both CD8⁺ and Jα18⁺ (NKT) cells from immunized animals without regard to immunogen. In contrast, CD8⁺ cells, but not Jα18⁺ cells, were necessary for the inhibition of late tumor growth (day 8). Thus, the developing tumor changes in sensitivity to NKT-mediated events and the role for NKT cells cannot be replaced by the presence of tumor-specific cells during early tumor growth. This suggests that recruitment/activation of Jα18⁺ NKT cells is an important consideration during the immune therapy of early stage tumors.