DNA from six avian sarcoma virus (ASV)-transformed mammalian cell lines was digested with the restriction endonucleases EcoRI, Xho I, or Sal I, fractionated by agarose gel electrophoresis, transferred to nitrocellulose filter strips, and hybridized with specific ASV [32P]cDNA probes. DNA from all of the ASV-transformed cell lines yielded three common virus-specific DNA fragments (2.4, 1.8, and 1.3 X 10(6) daltons) upon cleavage with EcoRI. Xho I appeared to cleave at least once within the integrated provirus and yielded a common fragment of 3.3 X 10(6) daltons as well as a second virus-specific DNA fragment whose size varied from 4.0 to 5.0 X 10(6) daltons in the different transformed cell lines. Sal I did not cleave within the provirus and yielded a single major virus-specific fragment of about 11 X 10(6) daltons in all transformed lines examined. Using specific cDNA probes, we show that the 1.8 X 10(6)-dalton EcoRI fragment contains sequences homologous to the 3' end of the viral RNA as well as to the src region of the viral genome. These studies clearly demonstrate that the same region on the ASV genome is utilized for provirus integration in different ASV-transformed cell lines.