The nucleotide sequences of several sites of recombination between adenovirus DNA and hamster, mouse, or human cell DNAs were determined. These sites of recombination had been cloned from adenovirus type 2 (Ad2)- or type 12 (Ad12)-transformed cells, from Ad12-induced tumor cells, or from a symmetric recombinant between Ad12 DNA and human cell DNA. One important precondition for the generation of recombinants between host and foreign DNAs might be the establishment of a chromatin configuration that permits access of foreign DNA and of the recombination machinery to cellular DNA. Such favorable chromatin structures might arise during cellular DNA replication or transcription or both. As a first approach toward investigating these more complex problems of foreign DNA insertion, we determined transcriptional activities of cellular DNA sequences at viral junction sites. The sites of linkage investigated in this study with respect to their transcriptional activities were those previously cloned and sequenced (W. Doerfler, R. Gahlmann, S. Stabel, R. Deuring, U. Lichtenberg, M. Schulz, D. Eick, and R. Leisten, Curr. Top. Microbiol. Immunol. 109:193-228, 1983). In addition, a site from cell line HA12/7 which is described in this paper was also analyzed. The results presented demonstrate that the cellular DNA sequences involved in linkage to viral DNA at five completely different sites in DNA from three different species are transcribed into RNAs even in cells which have not been transformed or infected by adenovirus. Some of these RNAs were cytoplasmic and were not poly(A)+. Human cell DNA sequences at the junction to Ad12 DNA in SYREC2 DNA were transcribed into poly(A)+ cytoplasmic RNA which could be translated in vitro. These results are consistent with the notion that at least some of the cellular DNA sequences at sites of insertion of adenovirus (foreign) DNA are transcriptionally active and thus provide an opportunity for recombination.