A non-replicating recombinant adenovirus type 5 (Ad5)-vector encoding the gene for interferon alpha-2b (IFNα2b) and the gene transfer enhancement agent Syn 3, with potential antineoplastic activity. Upon intravesical administration, recombinant adenovirus-interferon with Syn3 transfects both cancerous and normal bladder cells, and the adenovirus secretes interferon (IFNα2b) into the bladder. IFN exerts a direct antitumor killing effect and a bystander effect, thereby killing adjacent, non-transfected cancerous bladder cells. Syn 3 enhances the ability of the adenoviral vector to transfect cells.