We examined the integration site of avian oncornaviruses in the genome of different hosts with respect to the repetitive frequency of the cellular DNA sequences adjacent to the integrated proviral DNA. The following systems were studied: avian sarcoma virus (B-77) and avian leukosis virus (Rous-associated virus-61) in cultured duck embryonic cells and B-77 in cultured mouse 3T3 cells. These systems represent different host responses to viral infection, i.e., one in which both cellular transformation and viral replication occur (B-77-infected duck cells), one in which viral replication, but not transformation, occurs (Rous-associated virus-61-infected duck cells), and one in which transformation, but not viral replication, occurs (B-77-infected 3T3 cells). Two sequential hybridizations were used. First, large denatured DNA fragments (2.8 X 10(6) daltons) were reassociated to different C0t (mole-seconds per liter) values. Next, DNA remaining single stranded at different C0t values was isolated by hydroxylapatite column chromatography, immobilized on nitrocellulose filters, and hybridized with an excess of 3H-labeled 35S viral RNA to titrate the concentration of proviral DNA. Results show that B-77 sarcoma virus and Rous-associated virus-61 integrate in the unique region of duck DNA, whereas B-77 proviral DNA is associated with both repeated and unique host DNA sequences in transformed mouse 3T3 cells.