Rabbits tolerant to human immunoglobulin G were used to raise antisera against the Raji cell-bound circulating immune complexes from human breast cancer sera. After solid-phase adsorption treatment with glutaraldehyde-cross-linked normal human plasma, acetone-extracted normal liver tissue powder, and glutaraldehyde-fixed Raji cells, one antiserum reacted specifically with breast tissue extracts but not with extracts of other tissues, as examined by a counterimmunoelectrophoresis technique. Immunological reactivity of the treated antiserum was removed by incubation with normal, primary, or metastatic breast tumor tissue extracts. Incubation with normal human serum or extracts derived from tissues other than the breast showed no neutralizing effect on the antibodies. This specific antiserum reagent was used in a modification of the Raji cell radioimmunoassay. Raji cells were incubated with sera from cancer patients or normal controls and then reacted with 125I-labeled F(ab')2 fraction of the treated antiserum reagent. The amount of 125I-F(ab')2 bound was then determined. Although all sera exhibited elevated circulating immune complexes by the conventional Raji cell radioimmunoassay, 14 of 18 breast carcinoma sera demonstrated a significant uptake when compared with the normal population group as opposed to five (three lung and two colon) of 29 other cancer sera examined (p less than 0.001). An immunologically reactive breast tissue-associated antigen, purified from malignant breast tumor or normal breast tissue extracts with the use of antiserum reagent, exhibited an apparent molecular weight of 85,000 by sodium dodecyl sulfate; polyacrylamide gel electrophoresis and a pI value of 4.9 +/- 0.2. These results demonstrated that a breast tissue-associated antigen rather than a breast tumor-associated neoantigen, was involved in circulating immune complexes of breast cancer patients as detected by Raji cell immunoassay. It also implied the occurrence of disease-related autoimmunity in human breast cancer.