Transplantation of Sarcoma 37, infected in vitro with neurotropic-WS influenza virus (NWS), to virus-immune mice resulted in rapid disappearance of infective virus from the grafts and unimpaired tumor growth. NWS virus, present as a “passenger” in attenuated sarcomas, was also rapidly neutralized after transplantation of the tumors to vaccinated animals. Neoplasms thus released from the tumor-suppressive action of the virus slowly recovered their normal growth potentials. Administration of antiviral antibody, in the form of hyperimmune rabbit serum, to animals bearing infected Sarcoma 37 transplants completely neutralized the infectivity and tumor-inhibitory effect of NWS virus. The infectivity of the virus was not restored by retransplantation of these tumors to normal mice. Passive immunization produced a somewhat slower decline in the infective titer of NWS virus in Brown-Pearce carcinoma transplants. Infection of Sarcoma 37 grafts with the tumor-adapted FWS influenza virus also responded to treatment with homologous immune serum. In contrast to neoplastic tissue, passive immunization did not alter significantly the course of NWS virus infection of adult mouse brain and lung or subcutaneous homografts of mouse fetal lung and kidney. The results of these experiments will be discussed in conjunction with the studies on the Ehrlich ascites tumor presented in the following paper.