Many murine tumor models associated with murine leukemia virus(es) (MuLV) have been successfully treated by passive administration of antiviral antibodies. There is a large body of virus-negative tumors, however, which are lowly antigenic and thus refractory to such approaches. Therefore, an investigation was done for determination of whether such tumors could be rendered susceptible to passive serum therapy by introduction of MuLV antigens onto the tumor cell surface. For this purpose a 3-methylcholanthrene-induced fibro-sarcoma from inbred C57BL/6J mice was chosen. Following infection of the tumor in vitro with Friend murine leukemia virus (F-MuLV), the tumor was found to be susceptible to treatment with a high-titered heterologous anti-F-MuLV gp71 antiserum. The specificity of the treatment was determined by conduction of the therapy on the uninfected parental tumor, in which case there was no effect. In addition, therapy could be initiated at time points when demonstrable tumors were present and successfully treated animals were resistant to rechallenge with the infected tumor. Thus conversion of a lowly antigenic virus-negative tumor to an MuLV-positive antigenic tumor rendered such growths susceptible to immunologic management.