The potent antitumor activity of tumor necrosis factor (TNF) in combination with IFN-gamma can only be applied in local regimens due to their strong proinflammatory properties. It has been shown that the broad-spectrum matrix metalloproteinase (MMP) inhibitor BB-94 protects against TNF/IFNgamma-induced toxicity without blocking the antitumor effect. Here, we tried to explain this protective role of BB-94 and sought to assign roles to specific MMPs in TNF/IFNgamma-induced toxicity. By studying the expression of MMP genes in different organs and in the tumor, we observed that the expression levels of MMP-7, MMP-8, MMP-9, and MMP-12 and tissue inhibitor of metalloproteinase-4 are clearly up-regulated in the liver during therapy. MMP-8 and MMP-9 are also up-regulated in the lung and kidney, respectively. In the tumor, most MMP genes are expressed, but only MMP-3 is up-regulated during TNF/IFNgamma treatment. Using MMP-deficient or double-deficient mice, we have shown a mediating role for MMP-3 during TNF/IFNgamma treatment in tumor-free and B16BL6 melanoma-bearing mice. By contrast, MMP-12 seemed to have some protective role in both models. However, because most phenotypes were not extremely outspoken, we have to conclude, based on the set of MMP-deficient mice we have studied, that inhibition of a single MMP will probably not increase the therapeutic value of TNF/IFNgamma, but that rather, broad-spectrum MMP inhibitors will be required.