Abstract
Focus-forming virus can be induced by chemicals from virus-negative clonal lines of cells transformed by murine sarcoma virus. The viruses activated from such nonproducer cells have the host range and serologic properties of endogenous helper viruses of the cells rather than those of the sarcoma virus used to transform them. The evidence indicates that murine sarcoma virus-transformed nonproducer cells of two species contain the genetic information for both murine sarcoma and helper virus production in an unexpressed form.