In order to analyse immune-stimulatory effects of infection of human tumor cells with Newcastle Disease Virus (NDV), gamma-irradiated human breast carcinoma, colon-carcinoma or glioblastoma cells from defined cell lines were modified either by true infection with live virus or by cell surface adsorption of UV-inactivated replication deficient virus. Modification with live but not inactive NDV induced in all human tumor cells IFN-beta and the chemokines RANTES and IFN-gamma-inducible protein-10 (IP-10). In addition, infection by live NDV induced upregulation of HLA-ABC-molecules in all tumor lines tested and HLA-DR molecules in breast carcinoma lines. Two cell adhesion molecules, ICAM-I (CD54) and LFA-3 (CD58), were also upregulated on human tumor cells after infection with live NDV. When infection of MCF-7 breast carcinoma cells by NDV was performed in the presence of neutralizing anti-IFN-beta antibodies no upregulation of HLA molecules was observed suggesting an important role of IFN-beta in this process. Forty-eight to 72 hours after infection of the irradiated tumor cells with live NDV, many tumor cells were dead or in early or late stages of apoptosis. These results provide explanations for the function of the virus-modified autologous tumor vaccine ATV-NDV with which promising clinical results have already been obtained.