We have previously demonstrated that the growth of weakly immunogenic murine sarcomas leads to the induction of immunologically specific pre-effector cells in tumor-draining lymph nodes (TDLN). The in vitro activation of TDLN cells with anti-CD3 monoclonal antibodies (mAbs) and interleukin-2 (IL-2) resulted in the acquisition of effector function as measured by tumor regression in the adoptive immunotherapy of pulmonary metastases. Further studies were performed to characterize the mechanisms associated with in vivo tumor reactivity mediated by activated TDLN cells. By positive selection, CD4+ and CD8+ T cells were purified and activated by the anti-CD3/IL-2 method. CD8+, but not CD4+, cells manifested tumor-specific granulocyte-macrophage colony-stimulating factor (GM-CSF) and interferon-gamma (IFN-gamma) release in vitro, and elicited tumor regression in vivo. By contrast, only activated CD4+ were found to release significant amounts of IL-2 in response to tumor antigen but did not mediate tumor regression in vivo. Mixing the two purified populations enhanced the antitumor activity of the CD8+ T cells. In culture, IL-2 was found to augment the relative amount of tumor-specific release of GM-CSF and IFN-gamma by activated TDLN cells. We found that the tumor-specific release of GM-CSF and IFN-gamma by activated lymphocytes was strongly associated with the in vivo therapeutic efficacy of these cells. Evidence in support of this included the following: (1) cytokine release of TDLN derived after different durations of tumor growth correlated with tumor reactivity in adoptive transfer studies, (2) cytokine release of T cells derived from different lymphoid organs corresponded with tumor reactivity in adoptive transfer, and (3) in vivo administration of neutralizing mAb to IFN-gamma and GM-CSF significantly inhibited the antitumor reactivity of TDLN cells. These studies document the contributory roles of IFN-gamma, GM-CSF, and IL-2 released by activated CD4+ and CD8+ T cells involved in tumor regression.