Because CD4⁺ T cells play a key role in aiding cellular immune responses, we wanted to assess whether increasing numbers of gene-engineered antigen-restricted CD4⁺ T cells could enhance an antitumor response mediated by similarly gene-engineered CD8⁺ T cells. In this study, we have used retroviral transduction to generate erbB2-reactive mouse T-cell populations composed of various proportions of CD4⁺ and CD8⁺ cells and then determined the antitumor reactivity of these mixtures. Gene-modified CD4⁺ and CD8⁺ T cells were shown to specifically secrete Tc1 (T cytotoxic-1) or Tc2 cytokines, proliferate, and lyse erbB2⁺ tumor targets following antigen ligation in vitro. In adoptive transfer experiments using severe combined immunodeficient (scid) mice, we demonstrated that injection of equivalent numbers of antigenspecific engineered CD8⁺ and CD4⁺ T cells led to significant improvement in survival of mice bearing established lung metastases compared with transfer of unfractionated (largely CD8⁺) engineered T cells. Transferred CD4⁺ T cells had to be antigen-specific (not just activated) and secrete interferon γ (IFN-γ) to potentiate the antitumor effect. Importantly, antitumor responses in these mice correlated with localization and persistence of geneengineered T cells at the tumor site. Strikingly, mice that survived primary tumor challenge could reject a subsequent rechallenge. Overall, this study has highlighted the therapeutic potential of using combined transfer of antigen-specific gene-modified CD8⁺ and CD4⁺ T cells to significantly enhance T-cell adoptive transfer strategies for cancer therapy.