GB3 and GB5 are mouse monoclonal antibodies raised against human amnion. GB3 detects the epidermal basement membrane, and GB5 reacts with the junctional substances between the epithelial cells. These two antibodies were studied on breast tissues by indirect immunofluorescence. All (n = 8) of the normal mammary glandular basement membranes and epithelia reacted strongly with GB3 and GB5. In the intraductal carcinomas (n = 10), although nine out of ten tissues were reactive, their reactivities were greatly reduced. More strikingly, in the infiltrating carcinomas (n = 15), none of the tumor specimens were recognized by GB3 and only one out of fifteen biopsies was moderately reactive with GB5, indicating that the syntheses of the antigens of GB3 and GB5 were discontinued in the metastatic breast adenocarcinomas. These data suggest that these antigens may contribute to the interaction of the epithelium with the extracellular matrix and the intercellular binding between the epithelial cells; the loss of these antigens may facilitate the wide dissemination of tumor cells.