The product of the MUC1 gene, the polymorphic epithelial mucin (PEM), contains a large domain consisting of tandem repeats of 20 amino acids. Each repeat contains five potential sites for O-glycosylation, suggesting that this region forms a scaffold for the attachment of the O-linked carbohydrate which makes up more than 50% of the molecule. A number of monoclonal antibodies have been shown to recognize core-protein epitopes within the tandem repeat domain. One such antibody, SM3, reacts with the mucin expressed by breast carcinomas but shows little or no reaction with normal resting or lactating breast. Using primary mammary epithelial cells (HuME), an immortalized cell line derived from HuME (MTSV1-7) and a breast carcinoma cell line (BT20) the influence of the carbohydrate side chains on the binding of antibodies to core-protein epitopes has been investigated. We unequivocally show that the masking of the SM3 epitopes in normal breast epithelial cells is due to the carbohydrate side chains. In addition we demonstrate that the binding of two other antibodies (HMFG1, HMFG2) to core-protein epitopes is influenced by the carbohydrate side chains. The binding of HMFG-1 is particularly affected by sialic acid whereas the binding of HMFG2 is influenced by the length of the oligosaccharide side chains. Furthermore, inhibited elongation of O-linked carbohydrate side chains does not seem to interfere with the cell trafficking of the mucin to the cell surface.