The authors have used the indirect immunoperoxidase technique to examine the presence and distribution of milk fat globule membrane antigens in 12 cases of mammary Paget's disease using two monoclonal antibodies, HMFG-1 and HMFG-2. These stain breast epithelial cells but do not stain normal epidermis. The Paget's cells showed a similar pattern of cytoplasmic staining to that seen in the underlying intraduct or invasive carcinoma, therefore confirming them to be malignant ductal cells. To support this one of the cases was stained with the antibody LE 61 which is specific for nonepidermal epithelial cytokeratins. The result was strongly positive in Paget's cells in the epidermis but not in squamous cells.