In Vivo Replacement of the Helper Virus in Bryan's Strain of Rous Sarcoma Virus by AMV and RPL12 Viruses

Viremic chicks were obtained by intravenous inoculation of large doses of the following helper viruses in 11-day embryonating eggs: avian leukosis virus (RPL12), avian myeloblastosis virus (AMV), and Rous-associated virus (RAV-1). Bryan's “high titer” strain of Rous sarcoma virus (RSV) was then passaged 3 times consecutively in the wing webs of chicks made viremic to each helper virus. The RSV was titrated in the viremic chicks for each passage, and donor material for the next passage was taken from chicks inoculated with a limiting dilution of sarcoma virus. A stock of virus, differing from the original RSV and antigenically related to AMV (the helper virus) was produced by this procedure. It is concluded that the natural helper virus in Bryan's strain of RSV, i.e., RAV, was replaced in vivo by AMV and probably also by RPL12.