The O-glycans that decorate mucin glycoproteins contribute to the biophysical and biochemical properties of these molecules and hence their function as a barrier and lubricant on epithelial surfaces. Alterations in mucin O-glycosylation in certain diseases may contribute to pathology. It is known that both the host cell type and the amino acid sequence of the mucin tandem repeat contribute to the O-glycosylation of a mucin molecule. We expressed an epitope-tagged MUC1 mucin cDNA construct in the airway cell line 16HBE14o- and the colon carcinoma cell line Caco2 and used Fast Atom Bombardment Mass Spectrometry to evaluate the contribution of the host cell to differences in O-glycosylation of a single mucin. Many of the glycans detected on the MUC1 mucin were common to both cell types, as would be predicted from biosynthetic constraints. However, MUC1 synthesized in the airway cell line showed comparatively low levels of sialylation but carried a range of oligo-N-acetyllactosamine structures that were not seen in the colon carcinoma cell line.