A continuous line of BALB c embryo cells can be transformed by murine sarcoma virus (MSV). Of twenty foci selected at limiting MSV dilution, eighteen release both MSV and murine leukemia virus (MuLV). Two focus-derived lines, however, show no physical or biological evidence of virus production and contain no evidence of antigens of the murine sarcoma-leukemia complex. These lines have altered properties in tissue culture and in vivo and are morphologically indistinguishable from virus releasing MSV-transformed mouse lines. The addition of "helper" MuLV results in the rescue of the MSV genome with host range and neutralization characteristics of the MuLV used to rescue it. These findings show that virus production and release is not necessary for the maintenance of the transformed state in mouse cells and suggest that MSV is capable of initiating transformation without MuLV. In the nonproducer lines the sarcoma genome can be passed from cell to daughter cell for over 100 cell generations in the absence of any detectable virus expression. A preliminary report of some of these findings has been presented (Aaronson, 1970a).