Recognition of altered self-antigens in tumor cells by lymphocytes forms the basis for antitumor immune responses. The effector cells in most experimental tumor systems are CD8⁺ T cells that recognize MHC class I binding peptides derived from molecules with altered expression in tumor cells. Although the need for CD4⁺ helper T cells in regulating CD8⁺ T cells has been documented, their target epitopes and functional impact in antitumor responses remain unclear. We examined whether broadly expressed wild-type molecules in murine tumor cells eliciting humoral immunity contributed to the generation of CD8⁺ T cells and protective antitumor immune responses to unrelated tumor-specific antigens [mutated ERK2 (mERK2) and c-erbB2/HER/neu (HER2)]. The immunogenic wild-type molecules, presumably dependent on recognition by CD4⁺ helper T cells, were defined by serological analysis of recombinant cDNA expression libraries (SEREX) using tumor-derived λ phage libraries screened with IgG antibodies of hosts bearing transplanted 3-methylchoranthrene-induced tumors. Coimmunization of mice with plasmids encoding SEREX-defined murine wild-type molecules and mERK2 or HER2 led to a profound increase in CD8⁺ T cells specific for mERK2 or HER2 peptides. This heightened response depended on CD4⁺ T cells and copresentation of SEREX-defined molecules and CD8⁺ T cell epitopes. In tumor protection assays, immunization with SEREX-defined wild-type molecules and mERK2 resulted in an inhibition of pulmonary metastasis, which was not achieved by immunization with mERK2 alone.