Much interest exists presently in development of vectors for gene therapy of tumors based on RNA viruses because these viruses replicate in the cytoplasm and do not integrate into DNA. The negative stranded paramyxovirus, Newcastle Disease Virus (NDV) from chicken has the additional advantages of preferential replication in tumor cells and of oncolytic and immunostimulatory properties. We here describe the bispecific fusion protein alphaHN-IL-2 which binds to NDV, inhibits its normal cell binding property and introduces a new binding specificity for the interleukin-2 receptor (IL-2R). We demonstrate selective gene transfer to tumor cells expressing IL-2R via the bispecific fusion protein when using recombinant NDV carrying as marker gene the enhanced green fluorescence protein (NDFL-EGFP). Hemadsorption (HA) and neuraminidase activities (NA) of the HN protein of NDV were shown to be blocked by alphaHN-IL-2 simultaneously and the absence of HA-activity of modified NDV was confirmed in vivo. Retargeted virus-binding to IL-2R positive tumor cells was not sufficient for the process of cellular infection. It required in addition membrane fusion via the viral F-protein. By modification of recombinant NDV with a bispecific molecule, our results demonstrate a novel and safe strategy for selective gene transfer to targeted tumor cells.