The human adenovirus serotype 12 (Ad-12) EcoRI-C DNA fragment (0 to 16.5 map units) was cloned in the plasmid vector pAT153. This cloned Ad-12 EcoRI-C DNA fragment was subcloned generating recombinant plasmids which contained the Ad-12 SalI-C fragment (0 to 10.3 map units), the Ad-12 HindIII-G fragment (0 to 6 X 8 map units) and the Ad-12 AccI-H fragment (0 to 4 X 7 map units). Thus, we constructed recombinant plasmids which contain Ad-12 DNA sequences which represent all or part of the virus transforming gene region. The capacity of the cloned Ad-12 EcoRI-C DNA fragment to transform rat cells in vitro was assessed using the focus assay on primary cultures of rat cells. The specific transforming activity of this recombinant plasmid was in the same range as that found for intact Ad-12 DNA. Transformed foci which were induced by the cloned Ad-12 EcoRI-C DNA fragment were established as cell lines and the presence of Ad-12 DNA in these lines was demonstrated using the Southern blotting technique.