Protective immunity of BALB/c mice immunized with simian virus 40 (SV40) large T antigen (TAg) against SV40-transformed, TAg-expressing mKSA tumor cells is critically dependent on both CD8⁺ and CD4⁺ T lymphocytes. By depleting mice of T-cell subsets at different times before and after tumor challenge, we found that at all times, CD4⁺ and CD8⁺ cells both were equally important in establishing and maintaining a protective immune response. CD4⁺ cells do not contribute to tumor eradication by directly lysing mKSA cells. However, CD4⁺ lymphocytes provide help to CD8⁺ cells to proliferate and to mature into fully active cytotoxic T lymphocytes (CTL). Depletion of CD4⁺ cells by a single injection of CD4-specific monoclonal antibody at any time from directly before injection of the vaccinating antigen to up to 7 days after tumor challenge inhibited the generation of cytolytic CD8⁺ lymphocytes. T helper cells in this system secrete the typical Th-1 cytokines interleukin 2 (IL-2) and gamma interferon. Because in this system TAg-specific CD8⁺ cells secrete only minute amounts of IL-2, it appears that T helper cells provide these cytokines for CD8⁺ T cells. Moreover, this helper effect of CD4⁺ T cells in mKSA tumor rejection in BALB/c mice does not simply improve the activity of TAg-specific CD8⁺ CTL but actually enables them to mature into cytolytic effector cells. Beyond this activity, the presence of T helper cells is necessary even in the late phase of tumor cell rejection in order to maintain protective immunity. However, despite the support of CD4⁺ T helper cells, the tumor-specific CTL response is so weak that only at the site of tumor cell inoculation and not in the spleen or in the regional lymph nodes can TAg-specific CTL be detected.