The NS1 protein of influenza virus is a virulence factor that counteracts the PKR-mediated antiviral response by the host. As a consequence, influenza NS1 gene knockout virus delNS1 (an influenza A virus lacking the NS1 open reading frame) fails to replicate in normal cells but produces infectious particles in PKR-deficient cells. Because it is known that oncogenic ras induces an inhibitor of PKR, we addressed the question of whether the delNS1 virus selectively replicates in cells expressing oncogenic ras. We show that upon transfection and expression of oncogenic N-ras, cells become permissive for productive delNS1 virus replication, suggesting that the delNS1 virus has specific oncolytic properties. Viral growth in the oncogenic ras-transfected cells is associated with a reduction of PKR activation during infection. Moreover, treatment of s.c. established N-ras-expressing melanomas in severe combined immunodeficiency mice with the delNS1 virus revealed that this virus has tumor-ablative potentials. The delNS1 virus does not replicate in nonmalignant cell lines such as melanocytes, keratinocytes, or endothelial cells. The apathogenic nature of the delNS1 virus combined with the selective replication properties of this virus in oncogenic ras-expressing cells renders this virus an attractive candidate for the therapy of tumors with an activated ras-signaling pathway.