The transforming protein of Simian virus 40 (SV40), large T-antigen, regulates transcription both positively and negatively during the productive infection cycle. We have isolated a number of cellular genes which are expressed at elevated levels in SV40-transformed cells and have used these to study the mechanism or mechanisms by which the viral transforming protein regulates cellular gene expression. Small RNAs homologous to the mouse B2 repetitive sequence family are found at higher levels in transformed cells than in normal cells and we have shown that pure large T-antigen stimulates transcription of such repeats by RNA polymerase III. A class I major histocompatibility complex (MHC) gene is also activated as a result of SV40 transformation and we have used DNA-mediated gene transfer to study how this gene is regulated by large T-antigen.