SUMMARYTransformation of murine NIH3T3 fibroblasts with retroviral vectors carrying the mos, myc and the Ha‐ras oncogene, respectively, was associated with a strong reduction ofH2antigen expression in the cell membrane. Analysis ofH‐2Kand β2‐microglobulin promoter‐driven CAT activity in these oncogenic transformants and normal NIH3T3 fibroblasts revealed unchanged promoter activity, suggesting post‐transcriptional control of MHC class I expression by these oncogenes. Treatment with IFN‐gamma and TNF‐alpha caused 2‐ to 3‐fold enhancement ofH‐2Kand β2‐microglobulin promoter activity, as well as a normalization (TNF‐alpha treatment) or enhancement (IFN‐gamma treatment) ofH2membrane expression. These data suggest that IFN‐gamma as well as TNF‐alpha can counteract downregulation ofH‐2genes by interference with an oncogene‐induced, post‐transcriptional block as well as by a direc