Abstract The surface proteins of a lymphoblast line (SCRF 60 A) from an NZB mouse were studied. Iodination of the cell surface followed by solubilization and immunoprecipitation with antisera prepared against various MuLVs and cells produced a precipitate which gave a single peak in SDS-PAGE and comprised about 5% of the incorporated iodide. The antigen is a glycoprotein of apparent molecular weight (MW app) of approximately 70,000 daltons and comprises about 0.1% of the total cellular amino acids and about 10% of the cellular glucosamine. Studies on the oncogenic C type virus produced by SCRF 60 A, Scripps leukemia virus (SLV), showed that a glycoprotein of identical radioactive labeling properties and SDS-PAGE mobility is present on the surface of the virions. It constitutes about 10% of the virion amino acids and about 50% of the glucosamine. This protein reacts with sera which neutralize Moloney, Kirsten, Rauscher, AKR, and, of course, Scripps viruses. These data suggest that this is the antigen detected when murine leukemia cells are studied by immunofluorescence and that this antigen may be involved in virus neutralization.