We discovered an immunogenic 90kD glycoprotein tumor-associated antigen (TAA), and developed a murine monoclonal antibody based ELISA to detect TAA specific immune complexes (IC) in sera of breast cancer patients.For these investigations, Serum samples were randomly selected from 106 patients that had histopathologically proven breast cancer, and from 107 self-proclaimed and apparently healthy females.The serum samples were analyzed for the presence of the 90kD glycoprotein TAA-specific IC by a murine monoclonal antibody based ELISA.The murine monoclonal antibody, AD1-40F4, was developed to the autoimmunogenic glycoprotein TAA.This murine monoclonal antibody recognized an epitope that is different from those recognized by endogenously elicited polyclonal antibodies in the patient.The immobilized murine monoclonal antibody captured the in vivo formed IC that is present in a test sample.Capturing of IC was realized by enzyme conjugated goat anti-human IgG.The incidence of 90kD glycoprotein TAA-specific IC in sera of breast cancer patients was significantly higher than in self-proclaimed healthy females.Of the 106 breast cancer patients 67 (63%) were positive for the glycoprotein TAA specific-IC, as indicated by the normalized ELISA value above 0.410 OD (mean plus 3SD of normal) at 405nm.On the contrary, only 3 (2.8%) of 107 apparently healthy controls had positive ELISA value (p < 0.05 by Fishers test).Analysis of post-operative serum samples of breast cancer patients and correlation of results with clinical course revealed that 79% (34/43) sera from patients with recurrent disease were positive.This incidence was significantly higher (p < 0.05) than those patients who did not recur within 5 years (21%, 18/85).The results of this novel approach to a tumor biomarker assessment clearly denote a positive correlation between post-operative presence of TAA-specific IC and subclinical residual or recurrent disease in breast cancer patients.