The chicken anemia virus-derived Apoptin protein shows remarkable specificity; namely, it induces apoptosis in tumor cells, but not in normal diploid cells. We have exploited the Apoptin gene for use in cancer gene therapy. Here we demonstrate that adenovirus-mediated intratumoral transfer and expression of the Apoptin gene results in regression or complete remission of human hepatomas grown as xenografts in immune-deficient mice, and significantly increases their survival long term. Early after intratumoral injection, Apoptin could be detected in significant quantities by Western blot analyses and immunohistochemistry. Furthermore, cell death and disruption of the tumor integrity were apparent in the transduced regions. This experimental gene therapeutic strategy constitutes a unique example of specific antitumor activity using a virus-derived gene with broad-spectrum applicability.