Examination of syngeneic tumor regressor sera prepared by immunization of mice with several different lymphomas revealed a common pattern of reactivity to proteins expressed in these tumors. Antibodies present in these sera immunoprecipitate a triplet of proteins of 115,000 mol wt (p115), 80,000 mol wt (p80), and 32,000 mol wt (p32) from many but not all T cell lymphomas of mice. P80, the predominant molecular species immunoprecipitated with these sera, is a nonglycosylated, phosphoprotein that does not appear to be expressed at the cell surface. Comparison of the tryptic peptides of p32 and p80 indicated that the peptides found in p32 are a subset of those found in p80. Comparison of the tryptic peptides of p80 with those of the p120 gag-fusion protein of Abelson murine leukemia virus demonstrated that p80 and p120 did not share tryptic peptides. Comparison of the partial proteolytic products generated by treatment of p80 molecules from different tumors with V8 protease did not reveal heterogeneity in p80 among tumors of different strains of mice. Direct labeling and competition blocking experiments with lysates from normal cells failed to provide evidence of p80 synthesis in normal thymus, spleen, or bone marrow. Thus, p80 is a biochemically identified tumor-related antigen of mouse lymphomas.