Endogenous C-type viruses of BALB-c cells: frequencies of spontaneous and chemical induction.

A sensitive biological assay has been developed for studying the frequency of virus induction from mouse cells containing poorly infectious endogenous C-type viruses. A low spontaneous frequency of virus activation of one but not a second endogenous virus of BALB/c cells has been demonstrated. The resistance of BALB/c cells to superinfection by the more inducible virus appears to be absolute, whereas resistance to the other virus is only relative. Thus, the evidence indicates that the BALB/c cell exerts differential effects at two levels of regulation on the expression of its biologically distinguishable endogenous C-type viruses.