Plasma membranes purified from RBL-5 leukemia cells and solubilized with Na deoxycholate were fractionated on an Ultrogel AcA34 column. Fractions containing most of the tumor-rejection activity but low amounts of gp70 and p30 were consolidated and used to hyperimmunize semisyngeneic, CBF1, hybrid mice. Using the complement-dependent cytotoxicity assay, we determined the antibody titers against RBL-5 and several other leukemic cells. Only RBL-5 and EL-4 tumor cells were lysed. Similar results were obtained when the same tumor cells were used to absorb the syngeneic antiserum and the residual cytotoxic titer determined on RBL-5 target cells. Coupling of the gamma-globulins from this antiserum to Sepharose 4B facilitated the isolation of an antigen which inhibited the C'-dependent lysis of RBL-5 tumor cells and, when used for immunization, protected the recipient mice against a subcutaneous challenge of RBL-5 leukemia cells. By means of disc gel electrophoresis, in the presence of SDS and 2-mercaptoethanol, two polypeptide chains with molecular weights of 46,000 and 21,000 were resolved. The binding of the antigen to a lens culinaris lentil lectin column and its subsequent elution with alpha-methyl mannoside suggests that the specific tumor antigen is a glycoprotein.