We previously found that a minor fraction of single-stranded DNA (ss-DNA) isolated from native nuclear DNA of normal chicken embryonic cells and cells of other species hybridized with bulk nuclear DNA or cellular RNA in great excess. At least one-third of ss-DNA belonging to the nonrepetitious part of the cell genome could be hybridized to homologous RNAs. In the present work, similar results were obtained with ss-DNA from cells of chickens infected by avian myeloblastosis virus (AMV). To investigate whether this enrichment of ss-DNA in transcribed DNA sequences involves provirus DNA, radioactive AMV RNA and cDNA copies of AMV RNA were used. Most of the 70S AMV RNA hybridized much faster to ss-DNA from productively infected leukemic cells than to bulk DNA. cDNA, either double-stranded or single-stranded, made in the presence of actinomycin D hybridized to total nuclear DNA with similar kinetics. In contrast, about half of the double-stranded cDNA molecules hybridized 40-50 times faster to ss-DNA than to total DNA, indicating that only one of the provirus DNA strands seems to be present in ss-DNA. This was confirmed by the fact that relatively insignificant amounts of the ss-cDNA molecules made in the presence of actinomycin D could be annealed to ss-DNA as compared with bulk DNA. These results indicate that actively transcribed DNA sequences can be selectively distributed in the ss-DNA fraction, probably because of single strand breaks in the vicinity of transcription sites.The provirus theory of Temin (1), which postulates that the replication of RNA tumor viruses proceeds via DNA copies of viral RNA integrated in the host genome, has been substantiated by the discovery of successful transfection mediated by reverse transcriptase (2, 3), through an intermediate of viral DNA isolated from oncornavirus-transformed cells (4-6) and molecular hybridization between the viral genome and cell nucleic acids (7,8). In productively infected cells, the integrated provirus DNA appears to be more actively transcribed than most of the cellular genes (9).We have isolated, from the nDNA of various species, a minor fraction of single-stranded DNA (ss-DNA) and demonstrated that about one-third of the ss-DNA from cultured normal embryonic chicken cells (10) The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be-hereby marked "advertsement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.