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.