Replication-deficient adenoviral vectors have been widely used for gene transfer with the aim of delivering genes of interest to investigate their function and potentially to treat human disease. The ability to critically evaluate the biological role of a gene of interest, using adenovirus-based vectors, has been hampered by the development of local inflammation at the site of delivery. We have demonstrated that high multiplicity infection of human VSMCs with a replication-deficient adenoviral vector expressing no transgene leads to activation of the transcription factor NF kappa B. Activation of NF kappa B by this mechanism was able to augment gene expression from the human cytomegalo-virus immediate-early promoter (CMV-IEP) and induce expression of the adhesion molecule ICAM-1 in human VSMCs. These effects were inhibited by pretreatment with N alpha-p-tosyl1-L-lysine chloromethyl ketone (TLCK), a serine protease inhibitor known to inhibit the activation of NF kappa B. This important effect of the vector itself may have profound implications when replication-deficient adenoviral vectors are used for experimental gene transfer at a high multiplicity of infection.