Development of a new monoclonal antibody (mAb) MY.1E12 which reacts with sialylated MUC1 mucins is described. The mAb did not react with any component in the lysates of COS-1 cells, whereas it bound to sialylated MUC1 mucins produced by COS-1 cells transiently transfected with MUC1 mucin cDNA, strongly suggesting that the expression of the epitope of mAb MY.1E12 depends on the presence of the MUC1 mucin core peptide. The requirement of sialyl residues for antibody recognition was established by Western blotting analysis of extracts of various carcinoma cells and in situ desialylation. In all cases, the mAb binding of electrophoretically separated MUC1 mucin diminished after desialylation by mild acid hydrolysis. When Capan-1 pancreatic carcinoma cells were pretreated with benzyl-N-acetylgalactosaminide in culture, the MUC1 mucins produced under these conditions, which were detected by core peptide-specific mAbs, did not react with mAb MY.1E12. These results suggest that O-linked carbohydrate chains are important for the mAb binding.