The immunodominant DTR motif of MUC1 tandem repeat peptide has been assumed to be exposed during development of breast cancer due to a site-specific non-glycosylation. The binding of a series of DTR specific antibodies is strongly enhanced if the motif is glycosylated. Thirty monoclonal anti-MUC1 antibodies with a DTR specificy were tested for binding to this tandem repeat peptide and other (not shown) O-glycosylated with TF antigenic structure. In the peptides the threonine or serine residues are glycosylated to different degrees with the disaccharide Galb(1-3)GalNAca(1-O). Binding was measured in ELISA experiments by the groups of F. G. Hanisch (Köln) and S. Goletz (Berlin). The substances were identified by NMR spectroscopic techniques and MS analysis. Cancer Res. 1998, 58, 2541-2549.