Spleen cells from BALB/c mice bearing a syngeneic tumor (CSA1 M) 2 to 3 wk after inoculation with CSA1 M cells produced IL-2, IFN-gamma, and TNF upon in vitro cultures. This was previously demonstrated to be a result of collaboration between tumor-primed CD4+ T cells and APCs binding CSA1 M tumor Ags in vivo. The IL-2- and IFN-gamma-producing capacities decreased with the progress of tumor-bearing stages. This was parallel to the levels of IL-2 and IFN-gamma mRNAs expressed by cultured spleen cells. In contrast, comparable levels of TNF mRNA were expressed by all groups of cultured cells. However, large amounts of TNF were secreted by the cells from early but not from late tumor-bearing mice. TNF was produced mainly by the non-T cell fraction upon stimulation with CD4+ T cell-derived IFN-gamma. Therefore, the reduced TNF production by whole spleen cells from late tumor-bearing mice was restored by addition of rIFN-gamma to their cultures. Reciprocally to the progressive decrease in the production of IFN-gamma/TNF, the capacities of tumor-bearing mice to produce TGF-beta and IL-6 increased along with tumor growth. TGF-beta suppressed production of IL-2, IFN-gamma, and TNF, but not of IL-6. Moreover, IFN-gamma/TNF production was negatively regulated by IL-6. Taken together with the fact that the growth of CSA1 M cells is completely inhibited by the combination of TNF and IFN-gamma, these results demonstrate that the tumor-bearing state induces an abnormal cytokine network under which the production of antitumor cytokines is negatively regulated.