SummaryNIH Swiss mouse embryo cells transformed by polyoma virus were negative for virus and group-specific (gs) antigens of type C RNA tumor virus. When the transformed cells were transplanted into subcutaneous tissues of newborn NIH Swiss mice the tumors were positive for gs antigens but negative for infectious virus. Similarly, polyoma virus transformed hamster embryo cells led to hamster leukemia virus gs antigen after transplantation into newborn virus-free hamsters. Since mouse and hamster cells have been shown to contain the complete genome of the RNA tumor viruses, we suggest that a DNA tumor (polyoma) virus transformation in cell cultures and expressed (or activated) gs antigen of type C RNA tumor virus in tumors may be related events.We thank Mrs. S. Ricklis and Mr. J. Kim for their excellent technical assistance.