Abstract
The susceptibility of two classes of revertants of Simian virus 40 (SV40)-transformed 3T3 cells to retransformation by SV40 or murine sarcoma virus (MSV) was studied. Both serum-sensitive and density-sensitive revertants are not retransformable by SV40. MSV can transform both types of revertants. The MSV-transformed revertants grow to high cell densities and form colonies when suspended in semi-solid methylcellulose medium, but are unable to grow in 1% calf serum. The MSV-transformed revertants produce infectious MSV and murine leukemia virus and possess the same number of chromosomes as the untransformed revertants.