We have examined the mechanisms by which interferon (IFN)-gamma and IFN-alpha regulate the expression of 2'-5'-oligoadenylate synthetase (2-5A synthetase) and class I major histocompatibility complex antigens in murine T cells and in cell types of other histological origin. When treated with IFN-alpha both fibroblasts and T cell lines displayed a marked increase of the 2-5A synthetase activity and of the corresponding mRNA. The augmentation of the enzyme activity in T cells was induced by IFN-alpha at the transcriptional level, as determined by nuclear run-on analysis. In contrast IFN-gamma was capable of increasing 2-5A synthetase activity only in fibroblasts, but not in T cells. Nuclear run-on assays revealed that the 2-5A synthetase gene in T cells is not transcriptionally activated by IFN-gamma. After IFN-alpha and -gamma treatment we also observed a significant increase in class I gene expression in fibroblasts and T cell lines as measured both on the cell surface and by cytoplasmic RNA accumulation. In the case of the T cell line, DO1110, the observed increase in the steady-state levels of class I transcripts was a consequence of a high rate of H-2 gene transcription as demonstrated by run-on analysis. However, the molecular mechanisms involved in this IFN-dependent H-2 gene transcriptional activation are different between IFN-alpha and IFN-gamma. When the T cell lines DO1110, L12-R4 and EL4 were transfected with a plasmid containing a reporter gene (chloramphenicol acetyltransferase) under the control of a regulatory IFN-responsive DNA element of 237 bp or 1.4 kb, IFN-alpha was able to activate the transcription of these constructs. In contrast, IFN-gamma did not recognize the IFN-responsive element which, by itself, activated transcription of the reporter gene in response to IFN-gamma in other cellular types of non-T cell origin. Therefore, in the T cell lines examined, IFN-gamma increases the H-2 gene expression by acting on DNA elements located upstream of the regulatory segment used in this study or downstream of the cap site. This suggests a possible cell specificity in the activation of an IFN-responsive element, that in turn may regulate the IFN-inducible gene expression in a cell-specific fashion. Thus, the differential biological activities of IFN-gamma on T cells could be generated by a differential gene activation at the transcriptional level.