The viral DNA sequences in cells transformed by adenovirus type-12 (Ad12) DNA fragments were investigated by spot hybridization, a detection by autoradiography of nucleic acid hybrids formed between cell DNAs spotted on a membrane filter and various nick-translated Ad12 DNA fragments. In CY1 cell line, a rat cell line transformed by the EcoRI-C fragment (left hand 16%), all of the HindIII fragments included in the EcoRI-C fragment are shown to be present. In GY1 cell line, a rat cell line transformed by the HindIII-G fragment (left hand 7%), both of the BpaI-H and a part of the BpaI-J, two components consisting of the HindIII-G fragment, were found. A dominant presence of the Ad12 BpaI-H fragment (left hand end 4.5%) of the Ad12 DNA molecule, approximately 60% of the Ad12 transforming DNA sequences (Ad12 HindIII-G), was also shown in GY1. Spot hybridization used in the experiment will be of general use for detecting viral nucleic acid sequences in cells and provides a simple and useful screening method for investigating viral etiology of tumors and transformed cells.