We have previously shown that intravesical administration of adenovirus encoding human interferon α-2b (Ad-IFN) induced a marked regression of superficial human bladder tumors derived from cells that are resistant to over 1 million units/ml of IFNα protein in vitro. In addition, Ad-IFN appeared to produce strong bystander effects. In this study, we show that Ad-IFN causes marked inhibition of cell growth and apoptosis in cells of various tumor types, all of which are resistant to IFNα protein. In addition, strong perinuclear IFN staining was seen in all cell lines following Ad-IFN transfection and was never observed after exposure to the IFN protein. Ad-IFN induced proteolytic processing of caspases 3, 8 and 9, indicative of enzymatic activation. However, the caspase-8-selective inhibitor, IETDfmk, blocked apoptosis only in the cell lines that were sensitive to the IFNα protein and had minimal effect on Ad-IFN-induced caspase-3 or -9 processing and cell death, indicating that death receptor-independent mechanism(s) were involved in the cytotoxic effects observed for cancer cell lines resistant to the IFNα protein. Moreover, we document that a yet to be identified soluble factor(s) is responsible for causing the bystander effect observed following Ad-IFN treatment in IFN protein-resistant cancer cells.