Abstract
We used a monoclonal antibody to a high molecular weight glycoprotein of the milk fat globule membrane to study at the ultrastructural level the sites of accumulation of immunoreactive material in breast tumors. By light microscopy, the antigen was found only on the luminal membrane of normal resting or lactating breast cells. In tumors, antigen could be found in the cytoplasm. We used the avidin-biotin-peroxidase complex immunoperoxidase technique to localize the site of cytoplasmic accumulation of immunoreactive material in breast tumors. This technique showed that antigen is found on the membrane of cytoplasmic vesicles. Some of these vesicles were collapsed and gave rise to what appeared to be clumps of free cytoplasmic immunoreactive material. We have also documented that in some tumors the entire cytoplasmic membrane bears antigen, whereas in other tumors only areas of cytoplasmic membrane that form microscopic lumina express antigen.