We have used molecular hybridization and cDNA cloning techniques to isolate mouse cellular genes activated in SV40-transformed cells and we show that many of the clones belong to one of four sets. We characterize the cytoplasmic transcripts and genomic sequences homologous to two of these sets. The Set 1 transcription unit(s) is activated in all SV40-transformed cell lines analyzed, and experiments with tsA-mutant-transformed lines show that activation appears to require functional large T-antigen. This transcription unit(s) is also activated in mouse fibroblasts transformed by other agents, including retroviruses and chemical carcinogens. Activation of the Set 2 transcription unit(s) is more restricted, being confined to cell lines transformed by SV40 and retroviruses with distinctive biological properties.