Investigation of ErbB2 immunity in human breast cancer employing recombinant expression sources in immunoblot analysis revealed ErbB2-specific antibodies of the IgG isotype in sera of 14 of 71 cancer patients and 1 of 31 normal donors. Reactivity was confirmed on ErbB2-specific immunoprecipitates. Independent evidence of existing ErbB2 immunity was obtained after in vitro transformation of peripheral blood leukocytes from six positive patients. Furthermore, in vitro immortalization of B-lymphocytes unmasked existent ErbB2 immunity in 1 of 8 patients negative for ErbB2 serum antibodies. Determining shed ErbB2 extracellular domain as an indirect measure of tumor burden in ErbB2-positive malignancy, elevated serum levels were observed in 16 of 71 breast cancer and 1 of 31 normal donor sera. Strikingly, existing ErbB2 immunity correlated significantly with elevated shed ErbB2 ectodomain among the patients analyzed. Incidence of both ErbB2 immunity and elevated ErbB2 extracellular domain increased with a progressed disease stage and was significantly associated with metastatic breast cancer. These observations implicate soluble ErbB2 amounts in vivo in the development of ErbB2 immunity in breast cancer. They further project serum analysis of ErbB2 immunity and soluble ectodomain as potential markers of disease progression in ErbB2-positive malignancy.