Monoclonal antibodies were produced in mice immunized with proteins released into tissue culture fluid of human breast cancer cells maintained in vitro. One monoclonal antibody (SP-2) identified a Mr 90,000 antigen which appears to be a proteolipid. In immunoperoxidase assays, SP-2 reacted with 81 of 90 specimens of human breast cancer. It also reacted with 12 of 23 cancers of nonbreast origin but was unreactive with all normal tissues tested. The Mr 90,000 antigen, purified by immunoaffinity chromatography using SP-2, was used in an indirect binding inhibition assay for the detection of antigen in human serum. With this assay, 35 of 69 patients with breast cancer and 11 of 37 patients with benign breast lesions showed serum antigen levels above 6 units/ml. Patients with nonbreast cancers also demonstrated elevated levels of antigen in 32% of cases. The SP-2 defined Mr 90,000 antigen appeared to be distinct from carcinoembryonic antigen and other monoclonal antibody-defined breast cancer antigens of similar molecular weight. SP-2 may prove useful as a serum and/or tissue marker in breast pathology.

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Supported by C. N. R. Special Project “Oncology” Grant 84.00627.44.

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