We previously found that the level of Fas, a cell surface receptor for an apoptosis signal, increases at the mRNA level in influenza virus-infected HeLa cells prior to their death by apoptosis. Here we investigated the mechanism of activation of the Fas-encoding gene expression upon influenza virus infection. Nucleotide sequences for the binding of nuclear factor for interleukin-6 expression (NF-IL6), also known as CCAAT/enhancer-binding protein beta, were repeated 8 times in the 5'-end region of the human FAS gene, spanning from -1360 to +320. This region directed the expression of a downstream marker gene when introduced into HeLa cells and the activity of the FAS gene promoter was stimulated about 2-fold upon influenza virus infection. Gene expression driven by the FAS promoter was activated when human NF-IL6 was overproduced in a DNA when human NF-IL6 was overproduced in a DNA co-transfection study. Moreover, the DNA-binding activity of NF-IL6 increased after infection with the virus, whereas the amount of NF-IL6 seemed unchanged. The results suggest that NF-IL6 is activated upon influenza virus infection through post-translational modification and that the modified factor stimulates the transcription of the human FAS gene.