The use of gene-modified tumor cells as a strategy for active immunotherapy is currently undergoing intensive fundamental and clinical research. Most clinical trials use gamma-ray-irradiated tumor cells as vaccine, although little is known about the effects of irradiation on the immunogenicity of tumor cells. In particular, no data have been reported so far concerning the effects of gamma-ray irradiation on the expression of B7 molecules in tumor cells. In this paper, we show a neoexpression of the B7.1 molecule after gamma-ray irradiation in tumor cell lines from different tissues, while the B7.2 molecule remains unexpressed in all the cell lines tested. Furthermore, the induction of B7.1 molecule membrane expression after irradiation is shown to result from the neoexpression of B7.1 mRNA, and to be reproduced with H2O2 oxidative stress. These data could explain the enhanced immunogenicity of many tumor cells after irradiation, and could lead to new immunotherapy protocols.