Use of monoclonal antibodies (Mc 3 and Mc 8) prepared against human mammary-epithelial antigens of human mild fat globule membranes has enabled characterization of breast carcinoma (BC) associated antigens (BCAA), antibodies, and circulating immune complexes (CIC). For this study, BC patients were grouped on the basis of measurable tumor burden: Group I patients with no evidence of disease at sampling time; Group II patients with tumor burden less than or equal to 5 g; and Group III patients with known regional or distal metastases. In an in vitro simulation of tumor burden change, selected BC patients' sera were admixed with Mc 3 and Mc 8 at optimal concentration. CIC reduction (dissociation) for Groups I and III and increment (formation) for Group II were noted. Unlike Group I sera, Groups II and III sera required 4- to 16-fold dilution of Mc 3 and 4-fold more concentrated Mc 8 to achieve maximal CIC changes. Serum BCAA isolated by use of both Mc 3 and Mc 8 immunobead procedures showed apparent Mr 33,000 monomer, 66,000 dimer, and 95,000 trimer. When BCAA were added to BC patients' sera, autologous combinations resulted in small (7.7S) CIC for Groups I and III, and medium (9 to 12S) CIC for Group II. Conversely, allogenic combinations resulted in mainly small CIC for Group I, and intermediate CIC for Group II and Group III. Evaluation of circulating BCAA concentration by use of a three-step radioligand technique demonstrated significant discrimination between BC patients' sera (mean = 105 ng/ml) and normal control sera (less than or equal to 20 ng/ml). BCAA were found to be elevated in 31 of 46 (67%) Group I (mean = 70 ng/ml), 41 of 43 (95%) Group II (mean = 197 ng/ml), and 30 of 46 (65%) Group III (mean = 50 ng/ml) patients' sera, as compared to "background" levels in malignant melanoma and normal controls. Benign breast disease sera showed moderate BCAA increases (mean = 48 ng/ml) in 20 of 35 (57%) patients. Furthermore, serial sample determination of BCAA in 36 selected BC patients confirmed the above pattern, indicating that this assay can be used with some restriction to monitor tumor burden. Whereas in early breast carcinoma increase in BCAA concentration was concurrent with or antedated clinical objective evidence of tumor burden increase, significant decreased BCAA concentration was observed with tumor burden reduction. Overall, increased BCAA levels were associated with limited tumor burden (Group II) while decreased BCAA levels were observed with no evidence of disease (Group I) and known regional or distal metastatic advanced disease (Group III) during patients' follow-up.