CD4+ anti-tumor T cells reactive with rat adenocarcinoma 13762 kill tumor in vitro and cause regression of tumor in vivo. The role of various host immune cells in CD4+ T-cell-mediated tumor elimination in vivo was investigated by adoptive transfer of anti-tumor T cell clones to recipients that were selectively depleted of individual immune cell types. By these means, macrophages and NK cells were found to be required for tumor killing. Depletion of host CD4+ T cells, CD8+ T cells, or neutrophils was without effect on tumor elimination by anti-tumor T cells. An essential role for antigen receptor-negative NK cells is likely dependent upon secretion of IFN-gamma from NK cells since treatment of tumor recipients with anti-IFN-gamma antibody prior to adoptive transfer and tumor challenge abrogated T cell killing, resulting in progressive tumor growth. Viability of adenocarcinoma 13762 or anti-tumor T cells was unaffected by treatment with either IFN-gamma or anti-IFN-gamma antibody in vitro, but cell surface MHC class II expression was induced in tumor cells by exposure to IFN-gamma. In addition, tumor cells were isolated from tumor-bearing animals by absorption using anti-MHC class II antibody, demonstrating that 13762 tumor expresses cell surface MHC class II antigens in situ. However, if hosts were depleted of NK cells before tumor challenge, MHC class II+ tumor was not recovered. Collectively these results suggest that adenocarcinoma 13762 is eliminated by MHC class II-restricted CD4+ T cells by direct tumor killing.