trans-Activation of genes encoding activation-associated human T lymphocyte surface proteins by murine retroviral sequences. The mechanisms whereby RNA leukemia viruses cause T lymphocyte leukemias or lymphomas after a long latent period are not understood. We report here that infection of human T lymphocyte lines with a murine leukemia virus results in up-regulation of a number of lymphocyte-specific cell surface Ag. These proteins include CD2, CD3, CD4, the TCR, and MHC class I Ag. The expression of other cell surface proteins, such as LFA-3, are unaffected by the presence of the retrovirus. This up-regulation occurs at the level of the mRNA transcripts encoding these proteins, and is the result of increased transcription of the respective genes. The increases in transcription are the result of a trans-activation process by the leukemia virus. The transient introduction of chimeric genes consisting of MHC class I gene promoter sequences attached to the reporter gene CAT into human T cells containing murine retrovirus produces stimulated transcription of the reporter gene. Subgenomic portions of the murine leukemia virus containing the long terminal repeats and the 5' untranslated region are sufficient to produce transactivation of the same set of T cell genes as the whole leukemia virus. The finding that murine leukemia viruses enhance transcription and expression of a group of T cell surface proteins, all of which have been reported to be capable of transducing an activating signal to the lymphocyte, may be relevant to the pathophysiologic mechanisms whereby these viruses induce leukemias and lymphomas. 