Peptide vaccination is an immunotherapeutic strategy being pursued as a method of enhancing Ag-specific antitumor responses. To date, most studies have focused on the use of MHC class I-restricted peptides, and have not shown a correlation between Ag-specific CD8+ T cell expansion and the generation of protective immune responses. We investigated the effects of CD4-directed peptide vaccination on the ability of CD8+ T cells to mount protective antitumor responses in the DUC18/CMS5 tumor model system. To accomplish this, we extended the amino acid sequence of the known MHC class I-restricted DUC18 rejection epitope from CMS5 to allow binding to MHC class II molecules. Immunization with this peptide (tumor-derived extracellular signal-regulated kinase-II (tERK-II)) induced Ag-specific CD4+ T cell effector function, but did not directly prime CD8+ T cells. Approximately 31% of BALB/c mice immunized with tERK-II were protected from subsequent tumor challenge in a CD40-dependent manner. Priming of endogenous CD8+ T cells in immunized mice was detected only after CMS5 challenge. Heightened CD4+ Th cell function in response to tERK II vaccination allowed a 12-fold reduction in the number of adoptively transferred CD8+ DUC18 T cells needed to protect recipients against tumor challenge as compared with previous studies using unimmunized mice. Furthermore, tERK-II immunization led to a more rapid and transient expansion of transferred DUC18 T cells than was seen in unimmunized mice. These findings illustrate that CD4-directed peptide vaccination augments antitumor immunity, but that the number of tumor-specific precursor CD8+ T cells will ultimately dictate the success of immunotherapy.