Viral double-stranded RNA (dsRNA) and its synthetic analog poly(I:C) are recognized via multiple pathways and induce the expression of genes related to inflammation. In the present study, we demonstrate that poly(I:C) specifically induced the expression of matrix metalloproteinase-9 (MMP-9) in HaCaT keratinocytes. Studies using specific pharmacological inhibitors revealed the involvement of NF-κB, p38 MAPK, and PI-3K signal transduction pathways in poly(I:C)-induced MMP-9 gene expression. MMP-9 gene induction was sensitive toward treatment with the macrolide antibiotic bafilomycin A1, a vacuolar H+-ATPase inhibitor, and with the lysosomotropic agent chloroquine. However, cycloheximide treatment only partially blocked poly(I:C)-induced MMP-9 gene expression. Although HaCaT keratinocytes produce a number of cytokines and chemokines in response to poly(I:C), stimulation experiments revealed that exclusively TNF-α strongly promoted MMP-9 gene expression. During the antiviral response MMP-9 expression may be of importance for the tissue injury phase.