Five clones of rat kidney cells transformed by a small restriction endonuclease fragment of adenovirus 5 (Ad5) DNA (fragment HsuI G, which represents the left terminal 7% of the adenovirus genome) were analyzed with respect to the viral DNA sequences present in the cellular DNAs. In these analyses, the kinetics of renaturation of 32P-labeled specific fragments of Ad5 DNA was measured in the presence of a large amount of DNA extracted either from each of the transformed cell lines or from untransformed cells. The fragments were produced by digestion of 32P-labeled adenovirus 5 DNA with endo R.HsuI, or by digestion of 32P-labeled fragment HsuI G of adeno 5 DNA with endo R.HpaI. All five transformed lines were found to contain DNA sequences homologous to 75--80% of Ad5 fragment HsuI G only. Clones II and V contained approximately 48 copies per quantity of diploid cell DNA, clone VI about 35 copies, clone IV 22 copies and clone III 5--10 copies. These results indicate that a viral DNA segment as small as 5.5% of the Ad5 genome, contains sufficient information for the maintenance of transformation.