Transcription of the human adenovirus 5 genome in transformed rat embryo cells (DFK3) was investigated using two different approaches. Preferential digestion of transcribed viral sequences by DNase I was analysed using kinetics of renaturation of 32P-labeled Ad5 HpaI restriction fragments in the presence of material which was stable after nuclease treatment. The second approach was the hybridization of 32P-labeled nuclear RNA from transformed cells with Ad5 restriction fragments which were attached to a nitrocellulose filter. These two methods gave similar results. It was found that not all integrated regions of the Ad5 genome are active in transformed cells. 2,5 copies of the HpaI-E fragment of Ad5 DNA were found in transformed DFK3 cell line. Nuclear RNA from these cells hybridized to HpaI-E fragment of Ad5 DNA, but only about half of sequences of the integrated HpaI-E fragment was sensitive to DNase I digestion.