Abstract
Recently we demonstrated the induction of a C‐type RNA virus (XCIV) in tissue culture line XC derived from Wistar random‐bred rats. In this article two other rat C‐type virus isolates (NRKAV and RPLAV) are described. They were associated with, and isolated from, NRK and RPL lines originating in Osborne‐Mendel and Lewis rat strains, respectively. The main biological characteristics shared by the three viruses were buoyant densities in sucrose gradients in the range of 1.14 to 1.15 g/ml, endogenous DNA polymerase activity, high molecular weight (60 to 70S) RNA, rat major speciesspecific (gs‐1) antigen and gs‐3 interspecies antigen. Two of the isolates (NRKAV and RPLAV) were able to rescue mouse sarcoma virus (MSV) genome from a rat MSV‐transformed non‐productive line. The NRK line was assayed for virus release at various in vitro passage levels. In lower passages (43–73) spontaneous virus release was absent or minimal but the virus could be induced by treatment of cells with 5‐bromodeoxyuridine. At higher passage levels the cells produced a large amount of virus spontaneously, but in these treatment with 5‐bromodeoxyuridine decreased virus production. The results of karyological analysis suggested that clonal selection was taking place in the NRK line during consecutive in vitro passages and simultaneously with the process of increasing spontaneous virus release.