Immunization with Rauscher leukemia virus resulted in high antibody levels against the virustransformed malignant cell. As expected, antibody t o malignant cells followed immunization with standard virus preparations highly contaminated with cell membranes. Surprisingly, good antibody levels t o malignant cells also followed immunization with highly purified virus, electron microscopically free of cell membranes. The antibody response to malignant cells paralleled the antibody response t o the virus, under a variety of conditions. cell membranes, resulted in protection of the mice against subsequent challenge with malignant cells. In addition, good immunity t o malignant cells followed immunization with live, attenuated Rauscher virus, which was harvested from nonmalignant cells. This attenuated virus produced no malignancy after inoculation and presumably contains only viral antigens, and no malignant cell antigens.plantation antigen" on the surface of the Rauscher malignant cell. Rauscher virus is presumably integrated into the cell surface membranes which are, therefore, likely sites for transmission of cell enzymes and other proteins into the virus particle. This transfer of Immunization of mice with formalin-inactivated virus preparations, contaminated with These studies strengthen the evidence that Rauscher virus is the "tumor specific transFrom Surgery Branch and Viral Leukemia and Lymphoma Branch,