Previously we have demonstrated that a recombinant Newcastle disease virus (NDV) carrying the transgene EGFP can be retargeted to IL-2 receptor positive tumor cells by a bispecific fusion protein alphaHN-IL-2 in vitro. The purpose of the present study was to investigate the specificity and efficiency of gene delivery to tumor cells in vivo via this modified RNA virus. Prior ex vivo infection of murine lymphoma cells by the modified virus resulted in selective EGFP expression in IL-2R+ target tumor cells in vivo. Direct fluorescence microscopy and immunohistology showed viral replication in target positive tumor tissue resulting in much more EGFP expression than in target negative tumor tissue, 24 h after intratumoral injection of the alphaHN-IL-2 modified NDV. A quantitative real-time RT-PCR for EGFP mRNA. confirmed the selective gene expression in IL-2R+ tumor cells. Biodistribution studies showed that EGFP transgene delivery was reduced by 35-100% in liver, spleen, kidney, lung and thymus by the modified virus, while 98% of the transgene was delivered to IL-2R+ tumors. In conclusion, the modification of NDV by the bispecific protein does not compromise severely the efficiency of gene delivery into IL-2R-positive tumors, but greatly reduces viral gene expression in IL-2R-negative tumors and in normal tissues.