We have previously shown that adenovirus E1A proteins can block interferon-α (IFN-α)-signaling. In the current study, we examined if the same is true for IFN-γ signaling. Cotransfection experiments showed that both 289R and 243R forms of E1A could block the expression of an IFN-γ-inducible reporter gene. Similarly, in an E1A-expressing HeLa cell line IFN-γ failed to induce the synthesis of IRF-1 mRNA. This failure was due to a block in activation of the crucialtrans-acting factor, GAF, which in turn was due to the lack of IFN-γ-activated tyrosine phosphorylation of the STAT1α protein in E1A-expressing cells. The above defect could be attributed to a reduced level of STAT1α protein. The level of p48 protein, which is required for IFN-α signaling, was also lowered. However, the level of Jak-1 protein, one of the tyrosine kinases necessary for both IFN-α and IFN-γ signaling, was comparable in the E1A-expressing and the control cells. These results indicate that the observed inhibition of IFN signaling in E1A-expressing cells is a consequence of a lower abundance of the necessarytrans-acting factors.