SUMMARY In the present experiments, a neoantigen on the surface of tumour cells was separated in cell-free preparations. Large plasma membranes from cells of a recently induced murine ascites lymphoma were prepared by a two-stage centrifugal procedure using zonal rotors. The membranes appeared to be relatively uncontaminated and, when injected into syngeneic hosts, as little as 50 µg of protein/mouse induced tumor resistance in more than 50% of the animals. The new antigen, expressed on the cell surface, may be virus-dependent, but it appears to be incorporated into the plasma membrane as a cellular component. The specificity of the immunogenic effect is discussed.