Proteases are crucial for the spread of cancer cells from a primary tumor to the site of secondary growth. This study examined the ability of IFNgamma and TNFalpha to stimulate a better invasiveness in B16 murine melanoma cells, and investigated whether this enhanced ability was related to a higher expression of protease activities, such as urokinase plasminogen activator (uPA) and its receptor (uPAR), and matrix metalloproteinases 2 and 9 (MMP-2, MMP-9). We found that murine melanoma cells enhanced their lung-colonizing potential in vivo and invasiveness through Matrigel-coated filters upon costimulation with IFNgamma and TNFalpha; neither IFNgamma nor TNFalpha alone, at the dose used in the experiments, was able to elicit a change in the invasive/metastatic efficiency of melanoma cells. The invasive phenotype of murine melanoma cells stimulated with IFNgamma and TNFalpha was characterized by an enhanced uPA/uPAR and MMP-9 expression: TNFalpha promoted MMP-9 mRNA expression and pro-MMP-9 protein secretion, and the costimulation with IFNgamma and TNFalpha was required to potentiate the expression of mRNA and protein for uPAR, and to induce a redistribution of uPA from the soluble to the cell body-associated form. Both monoclonal antibodies, anti-uPAR and anti-MMP-9, caused a significant reduction of invasiveness in IFNgamma/TNFalpha-stimulated melanoma cells. These results indicate that invasiveness in B16 murine melanoma cells can be regulated in a cytokine-specific fashion and is dependent on the synergism between the uPA/uPAR system and MMP-9.