Interferon-gamma (IFN gamma) treatment of cells expressing the Ad12 E1A oncogene results in an increase in MHC class I antigen expression. The effect of IFN gamma upon both the steady-state level of class I mRNA and the in vitro rate of initiation of transcription of class I genes was studied in a rat cell line containing the Ad12 E1A gene. Results obtained showed a marked increase in steady-state level of cytoplasmic class I mRNA; however, a concomitant increase in the rate of transcription of class I genes was not observed. In contrast IFN gamma was shown to markedly increase the rate of transcription of class I genes in primary baby rat kidney cells.