The interaction of IFN-α and IFN-γ on the induction of antiviral, growth inhibitory, and (2-5)A synthetase activities was investigated in T98G and A549 cells. Synergistic or inhibitory effects which were host-cell dependent were seen with both simultaneous and sequential IFN treatments. Interestingly, IFN-γ1 pretreatment affected the levels of IFN-α-induced (2-5)A synthetase activity differently in the two cell types. In T98G cells, sequential treatment resulted in an overall decrease in induction of enzyme activity that was not observed in A549 cells. Receptor binding assays and measurements of steady-state mRNA levels indicated that this effect of IFN-γ pretreatment does not occur at the level of either receptor expression or induction of transcription of the low-molecular-weight form of (2-5)A synthetase.