A family of mucin-type glycoproteins, present in human urine, is coded by a single highly polymorphic gene locus PUM. We have previously shown that these glycoproteins carry epitopes recognized by a series of monoclonal antibodies, many of which were raised to the human milk-fat globule membrane, and which bind to a wide variety of carcinomas and certain normal epithelia. Here we show that in the normal human mammary gland, and in breast cancers the epitopes are present on the same family of molecules as that found in urine. Thus the genetically determined variation at the PUM locus accounts for much of the electrophoretic heterogeneity of the mucin-type glycoproteins present in breast cancer and serum from breast cancer patients that has been reported previously. Knowledge of this normal inherited polymorphism is essential to the interpretation of possible changes to these molecules in malignancy.