Factors affecting the frequency of transformation of rat embryo cells by simian virus 40.

The transformation of primary rat cells into established cell lines by eimian virus 40 has been monitared using the different restrictive assays of colony formation in sparse culture, dense colony or focus formation on a confluent cell sheet, and colony formation in semisolid medium. Primary embryonic rat cell cultures are considerably less susceptible to infection and subsequent transformation than the established mouse 3T3 cell line or later in vitro passages of rat cells. These embryonic cells show a stage-specific suaceptibiity to transformation but not to infection with a maximum susceptibility achieved at the 15th to 16th days of gestation. All transformed cell lines derived by SV40 infection of primary rat cells express viral T antigen as detected by immunofluoresence, though they differ greatly in their plating efficiency in semisolid medium containingmethylcellulose. Only the assay of colony formation in semisolid medium selects directly for transformants which plate well in that medium while all assays appear to select for cell lines containing viral T antigen.