The monoclonal antibody (MAb) NCRC-11 identifies an epitope expressed variably in human breast cancer. The degree of expression of this epitope in primary operable tumours is closely related to the subsequent clinical course of the disease (Ellis et al., 1985). The target antigen for NCRC-11 was isolated from subcellular membranes of breast carcinomas and purified by immunoadsorbent chromatography. NCRC-11 epitopes were expressed upon a large glycoprotein of more than 400 kd. This material was susceptible to degradation by pronase and papain and contained N-acetylglucosamine, as indicated by its binding to wheat-germ agglutinin. The NCRC-11-defined antigen expressed epitopes for the anti-human milk-fat globule membrane antibodies HMFG-1 and HMFG-2, and other antibodies against epithelial membrane antigens (EMA, LICR-LON-M8). The reactivity of these antibodies with tumour membranes was also similar, but not identical, to that of the NCRC-11 antibody. In competitive binding-inhibition assays, these antibodies partially inhibited the binding of 125I-NCRC-11 antibody to antigen, suggesting that the epitopes involved are topographically closely associated. Sandwich immunoassays demonstrated that NCRC-11 epitopes are likely to represent repeated structures of the NCRC-11 antigen. The findings presented are interpreted as indicating that the NCRC-11 antigen expresses a variety of epitopes which are associated with normal differentiation and malignant change.