A 175,000 dalton glycoprotein bearing a tumor-associated transplantation antigen (TATA) has been isolated from a Rauscher-MuLV-induced leukemia (RBL-5). The glycoprotein has the properties expected for a Friend, Moloney, or Rauscher-MuLV-induced TATA: 1) It is synthesized by RBL-5 cells and is expressed on the cell surface. 2) When inoculated into mice, it provides complete protection against challenge with RBL-5 cells at doses of 1 microgram or less. 3) The same protein, isolated from a Moloney-MuLV-induced leukemia, also protects mice against challenge with RBL-5. A similar glycoprotein is expressed on other tumor cells and on normal cells that do not express a TATA cross-reactive with RBL-5. Therefore, the immunogenicity of this molecule cannot be principally accounted for by either its unique or elevated expression on RBL-5 cells. A model is presented that accounts for these properties of the RBL-5 TATA. In addition, two other features of the RBL-5 TATA are discussed; an apparent abrogation of immunity seen only with certain doses of antigen, and a loss of the inherent immunogenicity of gp175 during purification.