This is the first report describing recombinant (rec) Newcastle disease virus (NDV) as vector for gene therapy of cancer. The gene encoding granulocyte/macrophage colony-stimulating factor (GM-CSF) was inserted as an additional transcription unit at two different positions into the NDV genome. The rec virus with the strongest production of the gene product (rec(GM-CSF)) was selected for our study. The insertion of the new foreign gene did neither affect the main features of NDV replication nor its tumor selectivity. The gene product was biologically active and stable. Tumor vaccine cells infected by rec(GM-CSF) stimulated human peripheral blood mononuclear cells (PBMC) to exert antitumor bystander effects in vitro in a tumor neutralization assay. These effects were significantly increased when compared to vaccine infected by rec(-) virus. Furthermore, rec(GM-CSF) led to a much higher interferon-alpha (IFN-alpha) production than rec(-) when added as virus or as virus-modified vaccine to PBMC. Two distinct cell types, monocytes and plasmacytoid dendritic cells were shown to contribute to the augmented IFN-alpha response of PBMC. In conclusion, the already inherent anti-neoplastic and immunostimulatory properties of NDV could be further augmented by the introduction of a therapeutic gene whose product initiates a broad cascade of immunological effects in the microenvironment of the vaccine.