Oncolytic virotherapy, a new type of cancer therapy involving viruses with oncolytic and immunostimulatory potential, is based on tumor selective viral replication, resulting in a specific lysis of tumor cells. Effective tumor targeting of oncolytic viruses remains a major problem because only a fraction of systemically applied viruses can reach the tumor tissue. We describe for the first time in an in vitro co-culture system that T lymphocytes can be loaded with Newcastle disease virus (NDV) in such a way that the virus load will be transferred to the tumor target cells upon contact of the T cells with tumor cells. The effectiveness of this NDV 'hitchhiking' on T cells can be influenced by the amount of virus, the ratio of T cells to tumor cells, the activation status of the T cells and by the virulence of the virus as shown by flow cytometry, quantitative real-time PCR and fluorescence microscopy. In a tumor neutralization assay in vitro, monolayers of human tumor cells could be completely and effectively destroyed by the addition of polyclonally activated human T cells loaded with oncolytic NDV. This process involves the formation of large T cell clusters as revealed by phase-contrast microscopy. Loading of oncolytic NDV onto activated T cells and adoptive transfer into a tumor-bearing host might enhance the efficacy of adoptive T cell therapy of tumors as well as tumor targeting of oncolytic viruses.